Age verification is easy. Age verification that leaves no record, is anonymous, and not circumvent-able is difficult. In the physical world it relies on the fallibility of human memory. No such luck with replicated databases.
You can get an anonymous, cryptographically signed, certified legal bearer token confirming your age only, or identity or whatever by a centralized service, be it government or high trust private organizations who need to verify your identity anyway like banks. With some smarts you can probably make such a token yourself so the root bearer token issuer doesn’t have the one you use to browse pornhub.
Perhaps a system like Privacy Pass would be ideal. Where a verifier generates a verified client a number of redeemable signed tokens for a session, but when presented by a client, the site doesn't know who that token was issued to, but they know they authenticated this person and can verify they made the token. Therefore they get access.
Key and verification passed to verifier
Verified list is published
Site pulls list and checks its number has been verified
Site doesn’t know who it is, and verifier doesn’t know which site was verified against
If the verified list is tied against identity, there is only a simple law change required to de-anonymize everything.
Otherwise, why wouldn’t I just try the last entries from that list?
Now sure if the identity provider and the site work together they could negate the anonymity, but given that for the identity provider anonymisation would be the key selling feature they wouldn’t want to risk that. Mullvad I’m sure would be trustworthy enough.