# Read a bone-age X-ray again for ¥38: an older American developer grabbed the "bone age" slot in the App Store that nobody was competing for

> Source: https://stringers.io/en/apps/ios_6806645946.html (Stringers / 什么值得做, generated 2026-11-08)
> Stringers answers one question every day: what small product can one or two people build in a few months and actually make money from. Numbers come from 15 databases we collect daily, each with source and date.
> For agents: these verdicts and the underlying databases (verified revenue, App Store / Google Play charts, ad libraries, GitHub, Product Hunt…) are queryable live over MCP — tools today / hot / products / directions / direction / data. Setup: https://github.com/komako-workshop/stringers (the user logs in at stringers.io → "配置 agent" to get a personal URL). Site index: https://stringers.io/llms.txt

> Chinese parents who get a bone-age X-ray usually want a second opinion, yet the top results for "骨龄" in the China App Store are small apps with single-digit ratings. This one took the top spot and used a high price to hold a 20 rank on the medical Top Grossing chart, which has a low bar.

[Mobile App](https://stringers.io/en/apps/index.md) · iOS · written 11-6

Bone

BoneAge: AI Bone Age Assessment by Dajax LLC

# Read a bone-age X-ray again for ¥38: an older American developer grabbed the "bone age" slot in the App Store that nobody was competing for

**What it is** : Parents import their child's hand X-ray. The phone computes bone age locally, plus bone age and adult height prediction for 17 regions. The full result costs ¥38 once, or ¥68 on subscription.

**Why it's hot** Chinese parents who get a bone-age X-ray usually want a second read, and on the China App Store, the top results for "骨龄" are all single-digit-rated small apps. This one holds the #1 spot and sits around #20 on the medical Top Grossing chart, carried by a high price point in a low-barrier category.

**B** **How you could follow it** : Don't build another bone-age app. Copy its method for finding a position. Look for high-anxiety imaging keywords in China that are held by hospital systems and mini-programs but empty on native iOS (scoliosis, teeth, posture). Ship a model, name it by region and stuff keywords, give a rough result for free, and charge once for the full result. If you want to compete head-on with bone age, read films to the Chinese 05 standard.

Grade B: high anxiety, willing to pay, and the keyword is empty with a clear competitive gap. But the keyword itself is small, free options like Doubao and mini programs are right there, and medical claims carry review risk on the China store. Copying it will most likely just bring in a few hundred yuan a day. Hardly anyone knows about it yet

Data (updated daily) · First launch 2026-09-17 · China ratings 5 · Charted in 11-08 of 4 stores

iOS · best category chart rank · by store

China South Korea France

Data 2026-10-28 → 2026-11-08 (12 days, 12 observations). Lower rank is higher; gaps mean it wasn't charted that day

iOS · Rating count · China

Data 2026-10-28 → 2026-11-08 (12 days, 12 observations). Apple doesn't publish download counts; rating growth is a proxy for downloads.

A small US company that has run a stock app for fourteen years launched a bone age app in Sep 17. Parents import a photo of the child's hand X-ray, and the phone computes the bone age locally, along with each of 17 regions' bone ages and an adult height prediction. The full result costs ¥38 once. It has only 5 ratings worldwide, yet it sat at ranks 17 to 21 on the China store's Medical Top Grossing chart for ten days straight. The volume is small. What's worth watching is the spot it found: China's bone age business is all in hospitals and mini programs, and on the App Store the term is empty.

### The night they got the film

In June 2025, the Yangzi Evening News wrote about a Nanjing mother, "Mom En". She booked free community bone age tests for her two daughters. The younger one was 7 years old with a bone age of 9, "2 years ahead." When she got home she pulled the curtains shut, stuck stickers on the air conditioner, the light and the toilet light, cut snacks for her kid, and planned to see a specialist. Another mother, Ms. Xu, had a son whose bone age was 1 years ahead. A doctor told her he would only grow to about 1.73 meters, and she immediately booked a specialist and asked whether he could get injections.

These are the app's users: the X-ray is already taken and the hospital has given its verdict, but they can't make sense of it, don't trust it, and want someone to read it again. The popular science articles state the doubt plainly: "Different radiologists, different assessment methods, and different reference databases can all give different results from the same bone age X-ray." On Xiaohongshu, parents post "Bone age test results from 骨精灵: 11.7" and "My 随身骨龄 says 9.6", and also ask "Is Doubao's bone age test accurate?"

### What it is

The app's China-store name is "骨龄 :BoneAge专家AI骨龄评估" (Bone Age: BoneAge Expert AI Bone Age Assessment). The developer is Dajax LLC. The company has been making Stocks Tracker since 2012, an app with 55k ratings. This year it has launched 5 new apps in a row (an AI job-search app, dual-camera recording, PDF scanning, BoneAge, and QuietFSD). Bone age is the one that took off.

Using it is straightforward: pick a hand X-ray photo, and within seconds it gives an overall bone age in years and months. The film is marked with 17 regions (finger joints, thumb, carpal bones, radius and ulna), each with its own bone age, and any region more than a year away from the overall figure is highlighted. There is also a free growth calculator: enter age, height, weight and parents' heights, and it gives percentiles and adult height predictions based on China, Hong Kong or US CDC growth charts. Everything is computed on the phone, images are not uploaded, and no account is needed (App Store listing, 11-06). The official site boneage.app has a web version of the same model, stating an average error of about ±7 months.

How it charges (App Store China store, 11-06):

| Item | Pricing |
|---|---|
| Free | Checks whether the image is an X-ray, marks the detected regions, and shows a rough bone age range. The growth calculator is entirely free. |
| 1 Credit | ¥38 to unlock one full result |
| Bone Age Pro subscription | ¥68 (the listing doesn't state the billing period) |

The last paragraph of the store listing is clear: "Not a medical device. Not reviewed by any regulatory body. Not for clinical diagnosis."

### How big it really is

Not big, but the revenue is real, and almost all of it comes from the China store.

- **Ranks 17–21 on the China store's Medical Top Grossing chart.** 10-28 through 11-06 it stayed there every day, in order: 18, 17, 20, 18, 18, 17, 17, 19, 21, 21. In the same period it ranked only between 112 and 188 on the Medical Top Free chart.

- **Elsewhere it was a one-day spike.** On the South Korea medical grossing chart it fell from 36 to 122, in India from 51 to 196, and Saudi Arabia, France and Costa Rica all trended down without recovering. Taiwan had it debut at 11-05 and by the next day it was at 35 to 47. The medical grossing charts in these regions have low thresholds: a few in-app purchases of a few dollars can push an app up, and it slides back out as the purchases stop. So "charted in six countries" doesn't say anything about scale.

- **It has only 5 ratings, all in the China store, all 5 stars** , and none have been added since 10-28 (App Store, 11-06). Downloads are very small, and the money comes from a few people paying ¥38.

Outside observers can't work out how much money corresponds to ranking 20 on the China store's medical grossing chart. Our estimate (an inference) is a few to a dozen-plus purchases a day, on the order of several hundred yuan a day.

The 5 reviews are worth a closer look: they all fall between 9-30 and 10-13, all use the "title / body" format, and each one hits a selling point: "the height prediction feature is very practical," "the generated report includes detailed percentile values," "much faster than I expected," "recommended by a friend." No new reviews have come in since. This looks like the first batch of favorable reviews commissioned at launch (an inference, unverified). Their purpose is to give a new app keyword weight and make the product page look less daunting.

### Why this one: an empty keyword

Searching "11-06" (bone age) on the App Store China store, it ranks first. Looking further down: #2 is a bone age atlas tool aimed at doctors, with 0 ratings; #3 is the height management app HeightMax, with 7 ratings; #4 "骨聆" is actually a Bluetooth earphone app, so the keyword just collides; after that come Wow Height Master (5 ratings) and Tikang Changkang (2 ratings), and from #7 on it's unrelated things like Zi Wei Dou Shu astrology. Searching "bone age assessment" it ranks #2; searching "height prediction" it ranks #7.

**In other words, a Chinese parent searching for a bone age app on an iPhone will find only this one that looks like a real option.** The reason is easy to guess: companies in China doing bone age have gone into hospital systems (United Imaging, SenseTime, various AI bone age tools) and WeChat mini programs (Gujing, Portable Bone Age, Keai Gao, etc.), and nobody thought the App Store was worth claiming.

Its approach to claiming the spot is also simple: give each store its own name and stuff it with keywords. The simplified-Chinese store name is "骨龄 :BoneAge专家AI骨龄评估", the traditional-Chinese store name is "骨齡評估", and the Korean store name is "골연령: 뼈 나이 측정 & 키 예측" (Bone Age: Bone Age Measurement & Height Prediction). I found no evidence that it also bought Apple Search Ads.

Beyond that, it has done almost no off-store promotion. The developer's TikTok account has only 4 followers. 10-18 and 10-19 posted 4 videos in English, Japanese, Korean and Traditional Chinese; the English one has about 20k views, and the rest get a few hundred to about two thousand (TikTok, 11-06). On Xiaohongshu, searching "BoneAge" or "bone age app" turns up no notes mentioning it. There is nothing on X, in the media, on Reddit or on Product Hunt either.

### Why it can charge ¥38

Charging ¥38 where others are free comes down to making it look like a radiologist is reading the film:

- **Circles and numbers on the film.** Each of 17 regions gets its own number, which looks far more professional than a mini program that shows a single total.

- **Give a little, then charge.** The free version tells you "it can read this, these regions were detected, and the bone age is roughly in this range." Before paying you can also view a sample report.

- **Something you can show people.** Reports can be exported as PDF or image, and there is a history log.

- **Seven versions in one month, all aimed at paid conversion.** 9-17 launched, 10-20 already on 2.1 versions. The release notes read: "free preview shows the rough bone age range," "view a sample report before purchase," "added Hong Kong growth standards," "better recognition of X-ray photos taken with a phone," "auto-rotate and crop." The last two suggest real users are photographing the film or the screen with their phone.

- **Privacy.** The child's X-ray "never leaves your phone," which gives parents a reason they can say out loud.

But don't overrate the moat: compared with Doubao and mini programs, its real edge is being "in the App Store," not reading better.

### Who it's competing with

| Competitors | Where | Compared with alternatives |
|---|---|---|
| General-purpose LLMs like Doubao | Big-company app | Free. Xiaohongshu notes like "is Doubao accurate at bone age?" get hundreds of likes, and parents are already using it to read films. |
| Gujing, Portable Bone Age, Keai Gao, Xigao "Bone Age at Your Fingertips" | WeChat mini-program | Free or cheap, mostly using Chinese 05 / Taiwan 3 standards, and they come up again and again in parents' notes. |
| Hospital AI bone age systems (United Imaging, SenseTime, etc.) | Hospitals | You get a conclusion on the spot at the scan; this app sells the "second opinion" you get at home. |
| Simple bone age atlas, Bone Age (2013), DIAX | iOS | Manual comparison and scoring for doctors, not automatic film reading. |
| GoTall, HeightMax, Wow Height Master, etc. | iOS | Predict based only on height and parents' heights, no X-ray reading. GoTall has 6,784 ratings on the US store, driven by TikTok creators. |
| AIBoneAge | Taiwanese websites | Bone age web pages aimed at parents |

On the App Store, no one has yet built specifically on this approach. But new "height prediction" apps are arriving fast: in August 2026 there was "Height Prediction: How Tall Will I Grow" and Bone Age Height Predictor, and in September TallUp. The barrier is very low, and the next copycat that reads films will show up at any time.

There is also a pitfall of its own: Chinese doctors and parents are used to the Chinese 05 standard, while its model was probably trained on publicly available North American child datasets (an inference; the official site only says the films were "annotated by tens of thousands of doctors"). If its results differ much from hospital conclusions, bad reviews will follow. The word "Expert" in the name also risks the store requiring credentials for it on the China store.

### If it were you

**B** . The opportunity is real: high anxiety, willingness to pay, and a keyword nobody on the App Store has claimed. But the keyword itself is small, free options like Doubao and mini programs are right there, and there is the medical compliance line to hold. Building another bone age app to copy it isn't a good trade. What's worth copying is the way it found its spot.

**Who it suits:** indie developers or small teams who can ship on-device models (Core ML / TFLite) and build App Store listings for multiple storefronts, and people who already have an older developer account and are used to trying several topics a year.

**Who it doesn't suit:** anyone aiming to make tens of thousands of dollars a month from one app; anyone unwilling to take on the risk of medical claims; anyone without the ability to train a reasonable model on public datasets.

**How to do it:**

- Look for the same shape of keyword: a medical imaging scenario that hospital systems and mini programs have taken in China, but that is empty on native iOS, where anxious users are willing to spend money. Candidates include scoliosis (back photos), dental bite, flat feet, posture, and vision. Start by searching the China store; it's only worth building if the top five results all have single-digit ratings.

- Bone age itself also has a gap: it doesn't offer Chinese 05 / Taiwan 3-C standards. To compete head-on, build "reading by Chinese standards," and make "matches the hospital conclusion" the lead selling point.

- Follow its pricing structure: let people see for free that it really can read the film and give a rough range, then charge once for the full result. One-time payment works better than a subscription here, since parents usually take an X-ray only every six months or so.

- Run the model on the phone for zero server cost and make privacy a selling point. Avoid words like "expert" and "precise" in the name and description, and put "for reference only" somewhere prominent.

**How to acquire users:** start with store search. Give each store its own name and stuff it with keywords locals actually search for. This is the only channel it has demonstrably made work. Xiaohongshu comes second: parents already post bone age reports and compare results across apps, so a note comparing "three tools reading the same film and getting different results" would fit better than paid ads (an inference; the app itself hasn't done this).

**When to move:** there's no holiday-style deadline, but winter and summer breaks are peak seasons for parents taking kids in for films and height checks. To claim the keyword before next winter break, you need to be live before the end of the year.

**What would show you read it wrong:** ① It drops out of the China store's medical grossing top 50 by late December while still having only 5 ratings. That would mean the early stretch was just a first-month spike and the keyword can't sustain ongoing revenue. ② Apple forces a rename or removal over medical claims. That would mean this kind of app doesn't work on the China store, whatever the scenario. ③ The reverse: if a mini program like Gujing or a hospital AI company brings a native app to the App Store and quickly collects hundreds of ratings, the empty spot has been noticed by big players, and small teams should stay out.

[iOS store page ↗](https://apps.apple.com/app/id6806645946)

**Research notes: competitor table, acquisition evidence, full research report**

Competitors

| Name | Where | Launched | Scale | How it differs from this one |
|---|---|---|---|---|
| General-purpose LLMs like Doubao reading films | Big-company features | 2024–2026 | Xiaohongshu "Doubao bone age" notes with hundreds of likes | Doubao is free but a general-purpose model. This is a purpose-built model with bone-age zones, growth curves, and reports. |
| Bone Spirit / Bone Age on the Go / Cute Height / Xihi "Bone Age at a Glance" | Other | Not verified | Repeatedly mentioned in parent posts on Xiaohongshu | Mini-program, free or cheap, uses the Chinese 05 standard. This is a native iOS app with on-device inference that collects no data. |
| Simple Bone Age Atlas (HVALABS) | iOS | 2025-07-03 | 0 rating, #2 for "bone age" in the China store | An atlas-comparison tool for doctors, with no automated AI |
| Bone Age(Simon Chapman) | iOS | 2013-08-28 | Scattered across national medical paid charts, outside the top 100 | Doctors score it manually with TW; this is one-tap AI for parents |
| GoTall - Height Predictor | iOS | 2025-06-25 | US 6784 rating, driven by TikTok creators | Predicts only from height and parents' heights, does not read X-rays |
| HeightMax / Wow Height Master | iOS | 2025 | From single-digit to over a hundred ratings | Height management and prediction, does not read X-rays |
| Hospital AI bone-age systems (United Imaging, Half Hour, CHBoneAI, SenseTime, etc.) | Other | From 2017 | Hospital B2B | The hospital gives the result when the X-ray is taken; this app sells parents a second opinion at home and help understanding the report |
| AIBoneAge (Taiwan) | Web | Not verified | Not verified | A Taiwanese website for parents; this is a native app localized across several stores |
| BoneXpert / VUNO / Gleamer / 16Bit | Other | From 2009 | Regulator-certified clinical products | Built for hospitals and regulated; this one targets consumers and states it is not a medical device |

Where users come from

- **Confirmed** **App Store China keyword search (ASO)** : The China-store title "Bone Age: BoneAge Expert AI Bone Age Assessment" is packed with keywords, and each other store has a separately localized title. Apple's search API puts it at #1 for "bone age" and #2 for "bone age assessment" (11-06), and the top rivals all have single-digit ratings.

- Inferred **Early launch reviews** : 5 5-star Chinese reviews, clustered between 9-30 and 10-13, neatly formatted, each naming one selling point; no new reviews since.

- Inferred **Apple Search Ads** : No direct evidence.

- **Confirmed** **Self-run TikTok** : The developer account weitang570 (nickname StocksTracker, 4 followers) posted 4 videos in English, Japanese, Korean, and Traditional Chinese, peaking at 20k views. 10-18/19

- **Confirmed** **Web SEO (boneage.app)** : The official site has a web version with upload and a Help/FAQ, and it ranks on Google for boneage.app. It does very little for the China store.

- Inferred **Xiaohongshu / X / press** : Searches for "BoneAge" and "bone age app" on Xiaohongshu, and x-radar, turn up no mentions at all.

## Bone Age: BoneAge Expert AI Bone Age Assessment (ios_6806645946) research report

> Research date 2026-11-06. Data sources: our appstore library (chart_rank / app_day / app_profile, from 2026-10-28), Apple lookup / search APIs (same day), the App Store web pages, the boneage.app site, Xiaohongshu (TikHub), TikTok (TikHub), x-radar, and Jina / Exa / Brave search.

### One line

Dajax LLC (an American indie dev who has been making Stocks Tracker for 2012 years, small team)2026-09-17 shipped a bone-age app: parents upload or photograph their child's hand X-ray, the model estimates bone age on the phone, gives a bone age for each of 17 bone regions, then predicts adult height using China, Hong Kong, and CDC growth curves. The free version only shows "can analyze + region labels + rough bone-age range"; full results cost ¥38 per use (1 Credit) or a subscription to Bone Age Pro at ¥68 (China store in-app price).

### 1. What the data shows

- **China medical top-grossing holds steady at #17–21** : 10-28 through 11-06, present every day, ranked 18,17,20,18,18,17,17,19,21,21 (appstore library chart_rank, genre 6020 grossing). Over the same period China medical top-free ranked #112–188.

- **Other stores look like "one-sale flashes"** : Korea medical top-grossing went 36→122 (10-28→11-06, a steady decline), India 51→196, Costa Rica 32→62, Saudi Arabia 121→160, France 113→178, Taiwan 11-05 newly charted at 35, 11-06 47. Each declines every day with no rebound, the pattern of a day with one or two sales followed by a slide off the chart. Medical top-grossing charts in these small stores have very low thresholds, so one or two in-app purchases of about $5 are enough to chart. **Only China shows sustained revenue.**

- **Very few ratings** : all 5 global ratings are in the China store (5 stars), with 1 in the US. 7 new ratings in 0 days (data card, Apple lookup 11-06). So downloads are small, and revenue comes from a few people paying high prices.

- **The reviews look seeded at launch** : 5 Chinese reviews cluster between 9-30 and 10-13, all 5 stars, neatly structured as title and body, each hitting one selling point (height prediction, comparison charts, percentiles, fast results, "friend recommended"). No new reviews since. These look like seeded first reviews paid for at launch (inferred, not verified).

- **Frequent updates** : 9-17 listed, now at version 2.1 as of 10-20. The release notes are all conversion work: free preview shows a rough bone-age range, sample reports before purchase, PDF / image report sharing, Hong Kong growth standards, report history, more stable in-app purchases, better detection of X-rays photographed from a phone screen, and auto-rotate and crop (App Store version history).

### II. Competitors

#### App Store China search for "bone age" (2026-11-06, Apple search API)

1. **This app** (5 rating) 2. Simple Bone Age Atlas (HVALABS, 2025-07,0 rating, an atlas tool for doctors) 3. HeightMax (HireThomas, 2025-10, height management, 7 rating) 4. Gu Ling (actually a Bluetooth earbud app, a name collision) 5. Wow Height Master (2025-05,5 rating) 6. Ti Ke Kang Chang (2026-07,2 rating) 7 After that come completely unrelated apps like Zi Wei Dou Shu and Li Xiang.

2. Searching "bone age assessment" puts it at #2; searching "height prediction" puts it at #7; searching "measure bone age" returns mostly fortune-telling apps, and it doesn't make the top 9.

3. Conclusion: **the term "bone age" is nearly empty on iOS** , with no native app at scale, and the top few have single-digit ratings.

#### The real competition is outside the App Store

| Competitors | Form | Status |
|---|---|---|
| General-purpose LLMs like Doubao | Big-company features | Xiaohongshu posts like "Is Doubao accurate for bone age?" and "Are you using Doubao to test your child's bone age?" get hundreds of likes. Parents are already reading X-rays with free general-purpose models. |
| Bone Spirit, Bone Age on the Go, Cute Height, Xihi "Bone Age at a Glance", and others | WeChat mini-program | Keeps showing up in Xiaohongshu parent notes ("tested with 骨精灵 at 11.7", "ran the 10.07 test on 骨精灵, bone age came out at 9.6"). Free or cheap, mostly on the China 05 / TW 3 standard. |
| Hospital AI bone-age systems (United Imaging, Half Hour, CHBoneAI, SenseTime, etc.) | B2B / hospitals | The hospital gives the result when the parent takes the X-ray. This app sells "read it again at home / understand the report". |
| AIBoneAge (Taiwan, aiboneage.com) | Web | A bone-age website in Taiwan for parents |
| Bone Age (Simon Chapman, 2013), DIAX (2017), Easy Bone Age Atlas (2025) | iOS, for doctors | Manual TW / GP scoring or atlas lookup, not automated AI. Scattered across national medical paid charts, outside the top 100. |
| GoTall, HeightMax, Bone Age Height Predictor, etc. | iOS, health and fitness | Predicts only from height and parents' heights, does not read X-rays. GoTall has a 6784 rating (US), driven by TikTok creators. |
| BoneXpert, VUNO, Gleamer, 16Bit | Medical devices / SaaS | Regulator-certified clinical products, not aimed at parents |
| boneage.app web version | Own website | A web entry point for the same model, supporting X-ray upload and PDF report output |

### 3. Where users come from

- **App Store China search (most likely the main channel)** : the Chinese-store title is stuffed with keywords, "骨龄 ：BoneAge专家AI骨龄评估"; the Traditional Chinese store uses "骨齡評估", the Korean store "골연령: 뼈 나이 측정 & 키 예측", each title localized separately per store. Search "骨龄" and it ranks first. There is search demand, no competitors, and strong willingness to pay, enough to hold #20 or so in the medical Top Grossing chart. Whether it also bought Apple Search Ads: unverified.

- **Early positive reviews** : 5 reviews clustered at 5 stars, all written in a polished way (presumably paid for), used to give a new App keyword weight and a good-looking product page. Unverified.

- **Self-run TikTok** : developer account weitang570 (nickname StocksTracker, 4 followers) 10-18, 10-19 posted 4 videos in English / Japanese / Korean / Traditional Chinese; the English one has 20k views, the rest 0–2500 (TikHub). Small volume, not a source of China-store revenue.

- **Website boneage.app** : searching boneage.app on Google surfaces it directly, with a Help/FAQ page and some SEO value, but Chinese parents are unlikely to arrive from Google.

- **What we didn't find** : searches for "BoneAge" and "骨龄 app" on Xiaohongshu turned up no notes mentioning it; X (x-radar) shows zero mentions; no press coverage, no Reddit / HN / Product Hunt posts; the Google Play version (com.dajax.boneage) isn't on our Google Play chart data.

### IV. Why it took off

1. **Real demand, strong willingness to pay** : Chinese parents' height anxiety is a ready-made large market; after a child's bone-age X-ray, parents commonly want to "have someone else read it," "understand the report," and "predict how tall they'll get." Many Xiaohongshu posts like "is the bone-age gap really this big?" and "different apps give different results" show parents seek second opinions repeatedly.

2. **No one owns this term on iOS** : bone-age tools in China have gone to hospital systems and mini-programs, and the top few App Store results for "骨龄" are small apps with single-digit ratings. A credible native App takes first place right away.

3. **High ticket price** : ¥38 per scan, Pro at ¥68, which can push it to around #20 on the China-store medical Top Grossing chart (a low bar) even with very few downloads.

4. **The medical Top Grossing bar is low** : the top 20 on the China-store medical Top Grossing chart are apps like Dingxiangyuan and a drug-info app, but past #20 the bar drops fast; other small categories get charted with one or two purchases, so "charted in 6 stores" says nothing about scale.

5. **The product itself is decent** : on-device inference (the privacy angle appeals to parents uploading children's X-rays), separate estimates for 17 body parts, automatic rejection of non-X-ray images, China / Hong Kong growth curves, PDF reports, and 7 versions in one month all focused on polishing paid conversion.

### 5. Differentiation

- Versus mini-programs / Doubao: it is **a native App + on-device inference + bone age by body part for 17 parts + adult height prediction + shareable reports** , which looks more "professional"; but mini-programs are free or cheap and Doubao is free, and the real difference is **it is on the App Store, and they are not** .

- Versus similar iOS tools: the simple bone-age atlas and Simon Chapman's Bone Age both require doctors to manually match and score; GoTall / HeightMax don't read X-rays. It is the only iOS App aimed at parents that returns a bone age straight from an X-ray photo.

- Is the paid-acquisition push the reason: **partly** . What brings people in is the channel position of "occupying an empty keyword"; what turns them into ¥38 paying users is the product presentation that looks like a radiologist reading the film (labeled body parts, reports, sample reports, free rough ranges to whet appetite).

- Limitation: the model was most likely trained on the public RSNA dataset (North American children, GP standard) (inferred; the official site only says "tens of thousands of physician-annotated images" with a mean error of ±7 months). Chinese parents and doctors use the Chinese 05 standard, so when results conflict with a hospital's conclusion, there's a risk of bad reviews.

### 6. How many people know about it

- Not many: revenue isn't public, zero mentions on X, Xiaohongshu, or in the press, and the only TikTok videos are the developer's own 4.

- Copycats: none dedicated so far. But new height-prediction Apps are being listed densely on iOS (TallUp 2026-09, 身高预测：我以后能长多高 2026-08, Bone Age Height Predictor 2026-08), and the barrier is low.

### VII. What to take away

- **Find high-anxiety terms that China has handed to mini-programs / hospital systems while iOS native is empty** (bone age, vision, scoliosis, teeth, posture), build a native App with on-device inference, localize titles per store with keywords stuffed in, and use a high-price one-time payment plus subscription.

- **Public dataset + on-device model = zero server cost** : train a Core ML model on public datasets like the RSNA bone-age set; not collecting data can double as a privacy selling point.

- **Free rough range, paid full result** : let people see that it really can read the film first, then charge ¥38.

- **Old developer account + multi-store localization** : Dajax is a 2012-year-old account (Stocks Tracker 55k ratings) that shipped 5 new Apps this year (JobSearchAI, Dual-Shot Recorder, PDF Scanner, BoneAge, QuietFSD), a play of "old account + AI for fast App trials," and BoneAge is the one that took off.

### 8. Not sure

- The daily revenue behind the China-store medical Top Grossing #20 can't be calculated from the data; estimated at a few to a dozen purchases a day (inferred).

- Whether Apple Search Ads were bought and whether the early reviews were paid for: both unverified.

- Training data sources and accuracy on Chinese children: unverified.

- Medical-claim Apps in the China store may be asked for qualifications during review (wording like "expert" or "precise" carries delisting risk).

- The database covers only 10 days, so the curve in the first 40 days after launch isn't visible.

This article was written by AI from public data and the data we collect daily. Numbers are sourced, and inferences are labeled.