It’s possible with certificates and 2fa issued by a government, which already have all your data, that would only verify that you are over 18.

We already have that in Spain, sort of. We have a government app where you have a digital id stored and you can make it create a verify qr that only shows if the user is over 18 or under 18, no more data. The qr only last 5 minutes active.

It is necessary? Not for internet access. That’s a duty of the one paying for internet in the household, not the government.