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.
I wasn't forgetting it. As it stands, at the OS level, you aren't supplying anything to prove your age. It's just a data field that software can read. And my point was that if that field, and social media was all it ever was, then, it's not great, but I can understand why the OP isn't too upset by it.
My point was more that it will never be just that.