alt Hacker NewsContinuous age verification isn't possible, so you'll have to store some sort of proof of age somewhere, and that proof will always be sharable.
Let's say Facebook has verified my age somehow. I could share my Facebook login credentials, or the token that their authorization server sends back in response. You can create some hurdles to doing that, like requiring a second factor, but I can just share that too.
You might as well go down the route of accepting that possibility. These systems are never going to hold up in the face of a determined enough teenager.
That really depends. A zero knowledge system would show to the verifier that the person is authorized for access _right now_, but thats just the answer to a particular challenge. Outside of the verifier who knows they came up with a random challenge without bias or influence, the response would mean nothing.
I think a lot of age verification systems are the solution to the real core of legislation - to make companies liable for underage viewing of content. To put such legislation in place without providing a feasible way to accomplish age verification would be argued as discriminatory.
In that sense, a zero knowledge system which doesn't give a company non-repudiation so that they can defend themselves in court may very well be insufficient. And that will require tracking identity long-term, although it could be done with a third-party auditor under break-the-glass situations with proper transparency.