Do you mean that I can get a permanent age-verification key from the third-party authority, then never have to contact the authority again (unless I want a new key)? If so (and assuming that zero knowledge proofs, which I'm not very familiar with, work), then my privacy concerns are resolved. (Well, I don't want the authority to keep a copy of my verification key, but FOSS code and client-side key generation should be feasible.)