

Funnily enough that is roughly the implementation the EU seems to be working on.
https://digital-strategy.ec.europa.eu/en/policies/eu-age-verification
On a side-note. I do not consider the government to be a trusted party. Whatever solution gets implemented needs to not provide the government any information that they can use for mass surveillance.
The two main requirements in my view are:
- The website that needs your age shouldn’t get to know your identity. They only get to verify your age.
- The government age verification shouldn’t get to know what service you are requesting access for. They only provide age verification.
Edit: You mention the certificate being short-lived, but one of the concerns mentioned in the proposed implementation for the EU age verification states that if that window is too short it can be used to determine identity.
All the more reason to download multiple copies