>valid proof would never become invalid proof

Somebody can give their proof of age to another person.

And? Presentation of someone else's valid credentials is not fixable by any privacy-preserving mechanism. You can set an expiration date in order to rotate them, and they can be fast-rotating.

In any case, it's a moot point: the correct amount of required identification is zero.

> Presentation of someone else's valid credentials is not fixable by any privacy-preserving mechanism.

And that is precisely why governments will never implement a privacy-preserving mechanism, which is exactly my point.

Compromised tokens would be trivially google-able within a day otherwise.