Countries like Denmark already have this. It's called MitID, and it a online government run sign in solutions used by banks, government institutions, online mar

Countries like Denmark already have this. It's called MitID, and it a online government run sign in solutions used by banks, government institutions, online marketplace as so on. I believe it already contains an age "claim", so you can get just the age of the person logging in and nothing else. It's built on OAuth/OAuth2 I think, so it should be fairly simple to add the age information if it's not already there.