First we're going to kill anonymity because that's the only way you can enforce any of these age bans, that as you show completely miss the point that these companies are bad for everyone.
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.
Your government probably doing this is as probable as the US government not starting a foreign war for Israel.