Can you give me one example of a 2FA implementation that would work? Where nobody in the login chain can know your identity, but you can still prove you're 18?

Every commercial 2FA app I know is intrinsically linked to verifying a known identity, so they do not provide anonymous verification.

Or they're hardware or secret key based, and their results can be shared and don't actually verify that the holder is 18+ (eg, physical authenticator). That works when your user wants to keep people out of their bank account, but doesn't work when they want to share their account online so everyone can login with it.

Edit: I forgot the third vitar requirement, that it needs to be sufficiently foolproof that the government can cheaply and efficiently implement and administrate it.

Can you give me one example of a 2FA implementation that would work? Where nobody in the login chain can know your identity, but you can still prove you're 18?

Every commercial 2FA app I know is intrinsically linked to verifying a known identity, so they do not provide anonymous verification.

Or they're hardware or secret key based, and their results can be shared and don't actually verify that the holder is 18+ (eg, physical authenticator). That works when your user wants to keep people out of their bank account, but doesn't work when they want to share their account online so everyone can login with it.