@a Here's my take:

Porn, if regulated further, should be regulated like so; using a porn site requires an email address and password, which it will check with the email provider to determine the user's age. If the person is underage, and the password matches with the email account, well, no porn for you. Case closed.

Only way you can watch it is if you know your email password and you're not underage.

(Also, this would require the account birthday to not be editable.)