I understand it doesn't need to be 100% correct. But I think what you're describing is either (A) going to be very privacy invasive, (B) going to create problems for lots of adults, or (C) going to be precisely as effective as a checkbox saying "I agree I am over 18 years old".

It's "social media". The whole thing is profiling for advertisers. The idea that there's privacy is laughable.

There is a network effect as a child's peers stop using social media though. Make it inconvenient enough for enough for kids and they'll read a book, take up slam poetry or whatever it was kids did before our attention became currency.