

Uh… if “it doesn’t have to be unique” then you may as well just have a password — everyone who knows that the password is “swordfish” is allowed into the adults-only club. There are things stopping people selling their actual paper-based passports en masse or just making photocopies. If you have an easily-replaceable digital token with no biometric info and it’s not tied to your identity in any way, there are no such constraints.
I’m still curious as to what it is that you have in mind. “Yes I have that card” will be communicated to random web services by the user presenting to them some kind of signed digital token I imagine, as is usual, and that token itself, or the user-held secret used in generating it, is what can then be sold, transferred, or used to track the user unless you have some way to prevent that. If you’ve given any hint of how you think it can be done, I didn’t get it.
One thing people sometimes think of is having the user be authenticated with a government (or other authority) server in real time whenever they want to prove their age to some stranger — but the system I saw which worked like that was obviously a pretty big violation of privacy so I assumed it wasn’t the sort of thing you meant. If that’s the idea, how would you prevent the central authority from keeping a record of when and where your “passport” was used?