Then you only know that someone is over 18. You don't know who is over 18. In particular, you don't know it's the person who's accessing your website.

Your comment sounds like a politician who has just heard a magic technical incantation that solves everything.

I'm sorry, you can't just drop "quantum computer", or "zero knowledge proof", or "flux capacitor", and think that you have solved the problem.

Just dropping "zero-knowledge proof" as if it's a mic drop moment is like when Turnbull said "the laws of mathematics are very commendable, but the only law that applies in Australia is the law of Australia".

The devil in all this is in the details. Just saying "zero-knowledge proof" is just barely more productive than saying "won't someone please think of the children?".

If you don't have a complete solution, then you're "not even wrong".

In addition to that, you have misunderstood how zero-knowledge even addresses the main problems. Not the technical problems, and DEFINITELY not the meatspace problems.