Kids are affected by image and media representations more obviously than adults. Where adults have established at least some sense of who they are and who they want to be, youth are generally less fully formed and still considering these things rather openly. As a result, kids respond strongly to image-based marketing, celebrity personas, and the like. Image, for kids, can be seen as a set of real possiblilities for self-presentation in ways that do not apply to adults.
I think there are more body-image disorders now than before. One reason is that with the proliferation of video technology, kids feel like they always have to look their best. They also feel like they are always on display. This can be dangerous for boys and girls both.