@dalias @Yuvalne @edwiebe @Em0nM4stodon Precisely - also as I described.
The one way around that would be storing the secret for the ZKP in a TPM.
Yeah, right, with that you can still run your own proxy and provide the ZKP for someone else.
But it is possible to then also use some forms of remote attestation so this doesn't work. Like, yeah, you can forward the ZKP, but then only you can decrypt the connection and not your "customer", as the decryption key is in your TPM and can't get out.
Despite all that, in worst case you can run a web browser in a VNC session for others to use, with your age claim. Nothing can prevent that - other than the ZKP not being actually ZK.
And that, indeed, is why ZKP aren't gonna happen for this. Even if they're cryptographically ZK, they'll end up signing more than just the age - at which point it's a privacy violation again and also no stronger than merely claiming your age in the first place.