I have a friend attempting to solve this. He's basically creating oauth for age verification. You sign up with his service and verify your age. After that it works similarly to oauth, but instead of return your identity, it just returns your age.

It's not a perfect solution, as he would still know who you are, but it's built in a way where you create a token locally to pass to the site that includes your age, and that site passes it to his site, which verifies the signature. So he knows who you are but not what sites you visit, and the sites know your age but not who you are.