Re: Age check = ID
"You can only prove it by publishing also your identity to every site you want to access"
Not true. The one sensible idea in this whole mess is having a third-party do the actual identity validation completely independently from anything else. They are then able to provide some sort of verification token to any site that asks for it, presumably based on some sort of username/password you provide, but without any connection to your actual identity. Even the verifying third-party doesn't actually need to hold on to any personal data, they can just check your age when you sign up and then immediately delete anything - it's not like you're going to get any younger so as long as they know the account owner is 18+ nothing more is ever needed in the future.
Obviously there are serious questions about whether any particular implementation will actually do any of this properly. And in the current case the fact that the biggest user of a verification system will also be the biggest third-party verifier raises issues even if they claim to keep everything separate. But it's certainly possible to conceive a system in which your age can be verified without ever needing to reveal your identity to the people wanting to verify your age, and even without having the connecting data saved anywhere to be hacked.