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.
Doesn't matter if they're the one proposing it, they're arguing in favor of it, that makes them part of the problem. People like that saying "well, this gross invasion of privacy isn't that bad for me" are why we have so much invasive bullshit going on in the first place.