Presumably you have an ID issued by your government. This same government can provide an eID, it's not like they don't know who you are. The whole question is:

Presumably you have an ID issued by your government. This same government can provide an eID, it's not like they don't know who you are. The whole question is: is it possible to prove your age to a pornsite without exposing your identity to said pornsite and without telling your government that you use it? And that would be the point of Zero Knowledge Proofs. Now of course, there are infinitely many poor ways to verify the age.