Er, you say sexual activity, but I was talking about sexting, which generally means partly or fully nude pictures of oneself—taken by oneself, alone—which don't depict sexual activity (except possibly masturbation). Those pictures will generally be classified as pornography—as my link said, they "constituted child pornography under relevant statutes according to respondents [law enforcement agencies]"—and producing them is an offense for which one can be and sometimes is arrested, and it is up to the police to decide not to enforce it (similar to speeding).

Then, if, say, someone still has naked pictures from age 15 (of herself in her bedroom alone) when she turns 18, and decides to start selling her own pictures online (or even posting them for free), and some guy buys them, and at some point the guy gets raided and the police find those pictures... I'm pretty sure the guy will get charged for possession of child pornography based on those pictures (assuming one can tell by looking that the girl was 15), and "no one abused this girl in the process" will not be accepted as a valid defense in court.