Natanael

You could tie it to requiring access to a digital ID (with password / PIN protection, etc), but yes kids could still “borrow” it

What you want is cryptographic Zero-knowledge proofs, not regular encryption. See anonymous credentials protocols.

And it does require every verifying entity to trust the issuer (each user could collect attestations from multiple issuers, to prove different things to different verifiers)

Another issue is the risk of deanonymization by verifiers simply asking for more proof of many different properties, until you can be identified anyway