the asker must know and trust the one providing the answer.

This is possible if there’s a central authority for that that everyone can agree to trust, like the government records directly. The issue is ensuring the rest of the chain remains anonymous so the only thing the authority gets is the request that an undisclosed service is verifying John Doe is 18+ and nothing else. And that’s not something many governments are going to want to allow with the increasingly alarming amount of authoritarian leadership.