Who has age authority? A state agency or service. Like the state issues an ID with age.

Preferable, we want the user to interact with a website, that website request age authentication, but not the website to talk to the government, but through the user.

Thus, something/somewhat like

1. State agency issues a certificate to the user
2. User assigns a password to encrypt the user certificate
3. User connects to random website A
4. Random website A creates an age verification request signed to only be resolveable by state agency but sends it to the user
5. User sends the request to a state service with their user certificate for authentication
6. State agency confirms-signs the response
7. User passes the responds along to the random website A

There may be alternative, simpler, or less verbose/complicated alternatives. But I'm sure it would be possible, and I think it lays out how "double-blind"(?) could work.

The random website A does not know the identity or age of the user - only to the degree they requested to verify - and the state agency knows only of a request, not its origin or application - to the degree the request and user pass-along includes.