Don’t get me wrong, I’m all for privacy. But between setting up the birthdate when creating my children’s local account on their computers, and having to send a copy of their ID to every platform under the sun, I’d easily chose the former.
I’d even agree to a simple protocol (HTTP X-Over-18 / X-Over-21 headers?) to that.
Tools should be provided if you want to do that but shouldn’t be standard. People should have freedom of choice on how to use their own property, in terms of computers, and how they manage/raise their children.