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.
OS defined does seem the best way, but I would prefer it wasn’t legislated. The people writing these rules have no clue about the real world, so they end up doing stupid things.