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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Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.