A government API. It gives claims. Such as user x is an adult.
User logs in first, authorises an app to get a specific claim a specific period or amount of times, User logs in to the app, app calls the API.
There is no need for IDs or credit cards or any of the sort.
The EU has already solved this years ago?
That's where you'd want an intermediary: you log in on porn site A => redirects to some intermediary which only needs to check your age => login from the state, get age, send user with verified token to porn site.
Porn site only knows your login and the fact you are an adult.
Intermediary knows someone needed to validate their age.
State knows this random no porn site wanted to check some claims about you.
Because what is the problem to solve here? If you do not require services to confirm the age of the user, they will not have to confirm the age of the user. This is a problem created.
I don't know if the commenter refers to this one or not, it's probably too recent and far from "solved".
The EU portal itself has an identity verification functionality, I can use my national ID (national insurance number, specifically) in EU polls, petitions and other EU portal functions, but that's anything but anonymous by design.