HExEr

The fun part is that you could do this extremely easily by just bundling a key with an item you can't buy unless you're 18. Like a beer that includes a PGP key, or a pornographic magazine with a page that's a randomly generated key.

The site/app doesn't need to know WHO you are, only that you have access to something you can only get by being 18. This way they're putting the burden of verifying your age onto someone else, someone else who is already legally required to do exactly that.

But no one is interested in developing this because the point of age verification has never been age verification. it's always been governments trying to tie a real-world ID to internet identities, and companies trying to get more private information on users so they can sell it to advertisers.