Perhaps one could construct a bond-based system. It wouldn't help family collusion, but it would help limit people of age selling their authentication ability.

Say you have a public service and a platform site (social media, gambling, whatever), and the user does authentication in a way that anonymously proves to the platform that they're of age while revealing nothing else, and without revealing to the public service what platform they're accessing. But the protocol requires some expensive data (token that provides access to a bond account) which anybody MITMing the protocol can obtain.

Then if Alice tries to sell her age verification abilities to Bob, the protocol could be designed so either Alice learns the negotiated key and can snoop on everything Bob does, or she has to let Bob do a man-in-the-middle over a channel and lose the ability to observe what's going on after the first key exchange; and then Bob can acquire the token and make use of it at a later time.

This is very handwave-ish, but I don't think such a protocol would be impossible to design.

Under normal use, Alice has no reason to drain her own bond account. But if she's selling to an anonymous crowd who might use the token at any time (hence she can't trace the traitor), some troll is eventually going to do it.