

I think you’ll need to generate a OTP type thing from a government site that’s a message with a timestamp and maybe the code provided (and chosen by) the website signed by that site’s private key, then have the 18+ website check that it’s signed by the government.
Basically, the digital equivalent of your teacher giving you a letter/consent form to bring home for mummy to sign and then return to the school.
Possibly you could have a fingerprint that goes on it that only the government could recognise if the website hands it over to them (likely under court order) if you are concerned about fraud/people using other people’s accounts.
🤨