"Jailbait", in the 4chan and reddit lingo, typically refers to non-pornographic images of underage women, often culled from social networking sites. If it was porn, it wouldn't be "jailbait", it would be "CP".
It's not "borderline-underage" (it's definitely underage), nor is it porn, so the title is rather misleading.
It doesn't have to be fully nude to be to some extent erotic. But we're arguing semantics here.
I didn't want to put "People are going to reddit looking for jailbait" in the title as that would imply that people are going to reddit to find actual jailbait girls in some kind of "to catch a predator" style scenario.