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.
I actually used this same example further up. Yes the GWOT made some terrible legislation that has done real damage, but it wasnt a slippery slope. They didnt make laws a little bit invasive but generally ok before slowly nudging it further until it got to the point where it was able to be used for ill. They went in hard and fast with abusable legislation which could be criticised for what it actually was, not what it would lead to in further legislation down the line (and it was criticised at the time).