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.
That isn’t a problem with proper implementation. Not that it will be done properly, just that it can be done properly
Trying to get every site that offers mixed content, or could do so to implement recognition of all of this is herding cats. It’s completely impossible without a walled internet. Moreover, there’s just no way a OS asking if you’re 18 is going to be accepted. It just isn’t. The expectation will be that they verify your age properly.