probably not 15. but mid 20s! and seriously, the 15 year olds rarely look 15 or even like teens. they're artificially aged via makeup and lighting. the younger the girl, the older she can be styled without looking too old. simple.
No it is not simple. I am well aware that models can be / are styled to look older and have commented on it in this very thread, however not every model can be styled and knows how to emote to project that image, take Karlie Kloss for example:
Hermes campaign - Karlie roughly looks her age which fits in with the fairytale theme, thumbs up;
Dior and Donna Karan campaigns - Karlie looks the part of a twenty-something young woman in both campaigns, thumbs up;
Oscar de la Renta campaign - Karlie's look and styling fits the blue outfit, but it does not fit the fur trim coat, neutral / thumbs down. Karlie is a good poser and emoter, but she could not be matured that much. In this case we are talking one picture, so I am not implying that the campaign was ruined, but a more sophisticated looking model would have pulled the look off better.
In recent seasons, Balenciaga seems to favor the awkward look, for the SS 2010 campaign (which I like quite a bit, graphically) apparently the folks over at Balenciaga thought that hunched shoulders and wide-eyed stares was appealing. Although Gisele has been cast for the upcoming campaign, so it appears that the Bambi trend is over.
This thread is about campaigns, but this really shows up in some runway shows with models between their awkward walks and scared facial expressions are not really selling the clothes.