This is step one. The next step is to say that it's not working and we need hardware attlestation or some such nonsense.

It helps soften people up for nastier stuff later.

As an example: Decades ago CPUID and a machine readable processor serial number was massively controversial. Today burned-in device IDs being sent to websites is status quo.

You don't think knowing a users self disclosed age-group would be beneficial to adtech? A) they shrug any liability for showing inappropriate things to minors when all the onus is on the OS to provide accurate info, and B) they get a highly-likely to be accurate piece of profiling information to add the the already huge pile they collect from spying on people 24/7 and harvesting microdata from browser APIs and mobile apps.