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.
My calculator doesn't need to know how old anyone is. Nor does my refrigerator. I suppose a case could be made for a router if you are all onboard for age gating everything privacy and freedom be damned. An OS isn't just Mac or Windows... the CA law is just so so dumb as written that I have zero faith in anything from Silicon valley.