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.
What would you say if, to be allowed to open your fridge (the one you own and purchased with your money) in order to pick something to eat out of it, you were required to make a blood test (say through some mechanism included in the door handle) to prove that you don't have any serious disease preventing you from, say, eating a slice of that delicious cake, or whatever?
The idea might not be completely stupid (one with some serious disease should not not eat what can harm them) but the implementation is not right. It's considering 100% of the population as suspects by default; just because they exist and have a fridge.
Replace 'fridge' with computer, 'cake' with online content, and 'disease' with 'being a minor'. That's how sad that is.