The thing is that as far as I can tell, a ZKP of age involves a state or similar attestor to issue an ID/waller that can be querried for age without revealing identity.

But attestor has to have certainty about the age of the person it issues IDs to. That raises obvious questions.

What states are going to accept private attestors? What states are going accept other states as attestors? What state won't start using its issues ID/Wallet for any purpose it sees fit?

This system seems likely to devolve national Internets only populated by those IDs. That can all happen with ZKPs not being broken.

That is how states work.