Hentai - Illegal in America?

So what's this about hentai being illegal in (North) America now? What constitutes an animated image being considered underage? Does it refer to teenagers as well as loli? And, if yes, what's the criteria for judging that? Help me out, world4ch, I need answers.

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In otherwords and damn hard to prove it illegal and possession is legal. No one wants to touch obscenity laws as they're against 1st amendment and not very clear.