that’s a lack of imagination….
just because i can’t think of a great solution doesn’t mean there isn’t one.
off the top of my head, here’s a system without a centralized surveillance database:

1. porn site gives you a random token,
2. you then hash that with a time-stamp,
3. get the token signed by your government ID system (like a dmv),
4. return the signed hash.
5. the government only knows that you used age verification for something somewhere….

now the hard part is just explaining cryptographic signatures and hashes to people….
of course people could still give a kid a verified account, but at least it’s harder for them to access it, i don’t need perfection….