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.
Will you be allowed to lie about the age? If yes, then it's a pointless law. If no, then whoever is checking needs to have more control over your device than you do, DRM style. That's gives them an entry point through which they can put whatever they want without you being able to control it.