Alot of this has to do with the artist's style as well. Anime/manga style characters look very youthful sometimes unless they are drawn a certain way, for example.
As an artist myself, when drawing young characters (original or otherwise) I make it a point to establish age in one way or another, or in several ways. If a character is young to the point where its hard to tell whether they are underage or not, but the artist still wants to draw the character, I think its well within reason to take a little artistic liberty and "age progress" the character by giving them a fuller body thats slightly to moderately less nubile. This can go a long way in discerning the age appropriateness of the character for the situation they are in. Sometimes its more the situation or theme the character is in thats the problem more than the character themself.
Artists and mods agree that great care must be taken in the sharing of this material, and to that end there are guidelines this site has and then there are the personal morals that the artist has themself. Coupled together they make a sound judgement. So in actuality, this isn't a problem that I see crop up often.