Indeed, working out the age of a person from a body can be difficult in real life (if you don't have experience of this go to a local night club and point out all of the 16,17,18 year old girls correctly, then find the 15 year olds pretending to be 16 etc... scary, our locals are normall mostly <18
). While we might not agree that a cartoon depicting child molestation etc is good right or whatever as long as the user does not actually act out their impulses what is the problem, thought generally isn't action.
Case in point, my avatar is technically 14 (in the series she is from), she is often depicted in rather risque situations though in fan art etc, I don't want to go to jail for trying to find a good wallpaper for my laptop!