i thought about this a while ago, there's a non privacy invasive solution that could work quite well:
at the age of 12 or 13, you're given a government code, like a hash which is able to generate OTP codes. this OTP code would provide proof of age for sites, which could also work in reverse to prevent predators infiltrating children's spaces.
You'd be sent them at two different ages, perhaps 13, then 18.
this hash would then be able to be used on a local device to generate OTP codes anonymously.
then, once you receive the 18 one, you can access adult only sites.