Well, I'd put those under Adult Content, but according to the guidelines, it says that that's primarily for "explicit sexual topics (questions/myTakes) made by Under 18 users"... but I think if you see comments that are too explicit, you can either go for Adult Content, or Explicit (""Posts should not contain detailed explanations of graphic or sexually explicit content"").
Oh man. You should come to High School, that would be a paradise to flag people cause honestly. Even the underage themselves talk about inappropriate stuff all the time.
Because the last thing gag needs is more censorship, the mods and admins would have a field day removing shit because "inappropriate" is subjective. It's a nice idea and I see where you're coming from, but the really worrying thing is the dudes who tread that grey area of creepiness.
Because they allow underage kids on this site. It's a free site, so, they can't tell little kids not to come