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.
Many OS here aren’t made by companies. They can all easily just be forked and edited to immediately wave people through any hypothetical age checks, moreover any website could just ignore it and wave everyone through.