The exact line gets blurry with various media. Kim, the female protagonist in the Broadway Musical "Miss Saigon" is a 17-year old whore. There's implied sex, as well as multiple stripper-dances throughout the musical to hammer in the sexual issues that Kim faces.
Something like Miss Saigon is allowed and not CP (despite being below the age of 18) because of cultural significance and whatnot.
There are clear cut cases of CP of course. But when musicals / movies / various media toy with the idea of 16-year-old or 17-year-old girls who are exploring their sexuality... where do you draw the line? What should get censored?
https://en.wikipedia.org/wiki/Lolita
Lolita the novel + movie also gets brought up a lot. As it is explicitly a story about a middle-age man exploring sexuality / erotic elements with an underage girl. It toes the line and never becomes sexually explicit, but its clear from the context what the novel is pushing.