The simplest possible such method? Single-use age verification codes, generated and validated by the government, sold on physical scratch cards with in-store verification of ID, piggybacking on the infrastructure we already use for selling alcohol and cigarettes.

This would be far easier to implement for websites too. You'd just have a single, unauthenticated API endpoint which, given a code, tells you if the code is valid (and marks it as used). Integrating with such an API is about 1 day of work for a competent dev. Even open, non-profit platforms like Mastodon could easily implement such a mechanism.

Scratch cards wouldn't have to be the only way of getting such codes. THe vast majority of people could just generate them in their banking app or whatever (which would still be far more privacy friendly than the current ID verification mechanisms).