I'm not suggesting that people actually authenticate to Pornhub using Login.gov's oauth, they would continue to auth (or not) as they do now. Login.gov can issue a token saying, in essence, "A user authenticated to me, and that user over 21, but I'm not going to identify them, I'll just give you a random GUID so this token will be unique".
edit to add more details, since I'm thinking it through: the token would need to include the issue date and be signed obviously, and would be ephemeral. Properly implemented, it could be done entirely in the browser (Firefox would have a "age verification provider" pull-down) in way that's transparent to the user and both private and secure. And since you have to be 18 to get a credit card, essentially any service you pay for with a credit card in your own name ought to be able to attest your age, even if it hasn't done KYC or scanned a government ID.