of course there's no perfectly privacy preserving solution for this, but ... zero-knowledge proofs have come a pretty long way. if I understand correctly it's

of course there's no perfectly privacy preserving solution for this, but ... zero-knowledge proofs have come a pretty long way. if I understand correctly it's possible to give 16+ people tokens and then they can make the signups (transactions with these tokens) and then check that the transaction is valid (that it came from some valid token without knowing which token), while also making sure that folks can't just fake spend someone's tokens -- this is how the new Monero version is going to work after all. https://www.getmonero.org/2024/04/27/fcmps.html https://www.getmonero.org/2024/04/27/fcmps.html Of course as others mentioned trading identities (tokens) is trivial. (As I expect not-yet-16 olds will start stealing identities/logins of older people.)