It's complicated. On the one hand, Anything <18 is a "child" for the purposes of the law, but nude does not automatically equal pornography. Technically (under federal law) to be child pornography it's got to have some kind of sex act, but this includes "lascivious exhibition of the pubic region", which gets pretty subjective. For better or worse, if there's no other "legitimate" reason for the nudity, it's probably going to be seen as child pornography. Perhaps ironically it's a bit easier to justify if you're an artist or a filmmaker than if you're just a teen selling your own nudes.