Info-minimized oidc handshakes with certified identity providers could verify age-category of a user with no other information shared.

Consider "log in with apple" as it is today. Depending on what you share, a relying website might not even get your name or email.

Yes, that was my thought as well when i was visiting UK and reddit kept asking me to verify my age. It might be even more private and non-trackable than that - if "age.id.gov" central authority effectively "provides a new random user id" (implementation may vary and does not need to have a "literal username") every time you try to use it / log into a website that needs to verify your age - this way websites can not even track you across platforms.

It seems like all the tech stack is there to implement a very simple and privacy-persevering solution.

It does not even smell of state censorship because a website does not have to check your age if it decides to be "non compliant".

Why isn't it implemented like that? Based on the comments it seems more like a "free-for-all implement-your-own-PPI-handling-thon".

This will ofc make life harder for a some groups of people - like people without / limited access to IDs etc. And i do not even argue that the whole thing is necessary.

But there seem to be vastly superior technical means to implement that, aren't there?

The only way to know that is to trust Apple.