Correct, but in the case of a website or a phone application, the producer of the application or website is also the provider. There is no reason why a pornography site could not be held criminally responsible for distributing pornography to minors.

The way we enforce this with alcohol and tobacco is that law enforcement runs a test were they attempt to purchase alcohol or tobacco from a store with a minor. Law enforcement can run a similar process where they attempt to access pornography(or other age restricted content) and if the site or application fails to restrict usage, apply crimial charges.

There is certainly public debate as to if we should use age restrictions on certain features of Facebook, but to say that enforcement is impossible, seems to contradict the data we have with other age restrictions.