Keira Knightley, Phoebe Cates, Thora Birch, Olivia D'abo, Brooke Shields, Michelle Johnson, and Mila Jovovich all did perfectly legal but still revealing nude scenes in mainstream movies before they turned eighteen.

And the reason I know this is because I did a paper on "sexuality as art versus sexuality as obscenity in cinema" when I was studying film, and the ages of some of the participants in nude scenes was a point of contention that I researched once I found the first one. Was it obscenity? According to the law, no, because it was presented in a non-sexual, non-lewd, non-violent, non-lascivious way. Presenting nudity on its own to advance the plot is considered artistically valid within film, but nudity presented during sexual contact, physical violence, or in the context of "unnatural acts" is considered potentially obscene.

Bottom line: As a filmmaker, you can show a naked 17-year-old in your movie if the context is framed properly (no physical contact, with some plot-worthy goal, even if that goal is to get a laugh or shock the audience (or another character) with a momentary flash of nudity), but you can't show a naked 17-year-old if she's having sex of any kind, or being attacked and/or tortured. Under those conditions it becomes pornography, which makes it illegal and obscene considering the age of the actor.

Strangely enough, you can show a fully clothed teenager being graphically dismembered, with as much gore as you'd like, and that's not obscene or illegal. Murder, maiming, and torture are just fine for underage characters in American cinema as long as no nipples, buttocks, or pubic hair are visible in the scene. However, if you were determined to display nudity, you could do it in a scene set prior to the attack, and as long as it had some value to the story, it would be legal and not obscene.

It's thorny and weird territory. I can't even imagine what it's like to be a censor or lawyer in Hollywood. I never went on to do anything with film, but the study of it did come into use in my (limited) writing career.