Most respectable porn sites require ID verification for uploaders of anyone in the scene. It can be faked but its not as easy as one would think with the better verification services. If its found to be a minor or something slips through, the FBI is made aware. They can't go after all of them so, so only the worst are actually tracked down.

Owning a porn site that hosts original content with no verification at some point will be criminally/civilly liable under US law. You are required to make a reasonable effort. Even if they don't get that far into trouble, no respect website hosting company will tolerate it.

You tell them yo dude this site is carelessly hosting child porn here is the proof. Site suspended fast and they are given a chance to fix it. They don't get many chances and eventually the host will just ban them. There hosts that will look the other way but they wont offer good enough performance or will be insanely expensive.