Atleast some EU countries have ”strong authentication” services, which work like Google login, except there’s a network of trusted providers - like banks or telecoms.

To get started, you need get an account at one of the providers by physically visiting their store or office and show your ID. With their account you can sign up to the services or other auth providers fully online. Auth providers use 2-factor auth (typically website username + phone app), but they also include physical sim cards or key generator devices…

Anyways, while it’s currently used to pass some required personal info to the other party (name, social security number), it could propbably be changed to send just one bit: adult true/false.