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.
Yes, this is precicely why a client side flag is so much better than a server side "send your ID documents to a third party" system. As I said, the Californian bill seems to have no requirement that the OS account creation process actually checks the value entered, just that it requires it at setup.