Oops! I read Groups, and my brain went
Groups.
Like Jorick said, this can get to be a pretty grey area. Sexual
themes, are allowed, if they are just implications of sex. There's more to a sexual theme than straight up, written sex. But no sex scenes should be written between adults and teenagers. So, if you are intending to have explicit sex scenes in your roleplay, save yourself a lot of trouble and just stick it in the Libertine forum. If, however, your sexual theme is limited to references to sexual acts and innuendo, then there is no need to filter age groups because there is nothing illegal there.
Theoretically, any roleplay in the majority Groups section should be open to both teenagers and adults. Sometimes GM's say 18+ because that is simply the age range they prefer writing with, but if they are consistently roleplaying material that is unsuitable for -17's to join, it should be in the Libertine forum.
The grey area starts arising when there is no intent for sex, but sex happens naturally through roleplaying. Then, it is not permissible for the scene to be between an 18+ and a 17-, but that doesn't mean that there can't be both age ranges within the roleplay.
If this isn't clarified enough for your tastes, why don't you give me the specific example that brought up this question.