Well, we definitely do not allow images of children on Elliquiy, whether realistic (photos) or drawn (anime, cartoons, manga, etc.).
For other images, though, people just need to use common sense. All humans in posted images need to appear to be eighteen or older. This mainly applies to stuff like anime, not so much photo-realistic images (but it should still be kept in mind, regardless).
For example, Character X from Anime Y might be listed as twenty, but if she looks like a prepubescent girl, it will be counted as an image of an underage character and removed.