Imagine you have a private key whose public key is signed by a trusted issuer if and only if you're an adult.
Now you use this to get an account. What if you share your private key? Well you can only have one account per key and it's otherwise the same as just obtaining the porn for the improper users yourself (which is always possible).
Won't that ID just get linked between sites and used to track you? Yes. But you can use a somewhat more complicated version of this scheme where you can get access to a single unlinkable ID per site... so then you can't be tracked.
Imagine you have a private key whose public key is signed by a trusted issuer if and only if you're an adult.
Now you use this to get an account. What if you share your private key? Well you can only have one account per key and it's otherwise the same as just obtaining the porn for the improper users yourself (which is always possible).
Won't that ID just get linked between sites and used to track you? Yes. But you can use a somewhat more complicated version of this scheme where you can get access to a single unlinkable ID per site... so then you can't be tracked.