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.

Reminds me of how people wanted to ban rock and roll in the 50's.
"Oh noes our family values are being attacked!"