You're wrong about that last as a matter of law (at least in the US). Anyone creating a sexual image of a subject who is 17 or younger is guilty of creating child pornography, no matter what the circumstances are. I'm sure that if Congress had considered the case of 17-year-olds taking pictures of themselves they would have written the law differently, but they didn't. One would also hope that prosecutors would exercise discretion about prosecuting these cases, but while some might others certainly don't.