I feel like you could, theoretically, have a service that has an ID (as drivers license ID), perhaps operated by your government, that has an API and a notion of an ephemeral identifier that can be used to provide a digital attestation of some property without exposing that property or the exact identity of the person. It would require that the attestation system is trusted by all parties though, which is I think the core problem.

Wouldn't this require the API provider to know that tbe citizen is connecting to the app? Grindr users might be squeamish about letting the current US admin know about that.

ICAO compliant ID cards (aka passports) and many national ID cards already are smartcards with powerful crypto processors.

Hand out certificates to porn, gambling or whatever sites, that allow requesting the age of a person from the ID card, have the user touch their ID card with their phone to sign a challenge with its key (and certificate signed by the government), that's it.

Government doesn't know what porn site you visited, and porn site only gets the age.

Not necessarily, you can define the protocol such that it's all done with opaque IDs instead of identifying info.

This is not only theoretical, the German ID card ("elektronischer Personalausweis") can do exactly this.

Do you feel this way when you enter credit card information when making a purchase online?

Yes.

> Transferring your age and a way to verify it to any third party is by definition a privacy violation.

No it's not. Unless...

> Doing so in a safe way is literally impossible since I don't want to share that information in the first place.

...well then it is.

But it's not constructive to claim that proving your age to someone is by definition a privacy violation. If someone wants to prove their age to someone, then that's a private communication that they're entitled to choose to make.

It is true that if technology to achieve this becomes commonplace, then those not wishing to do so may find it impractical to maintain their privacy in this respect. But that doesn't give others the right to obstruct people who wish to communicate in this way.