The prime onset of eating disorders is adolescence. Why? No one knows for sure. Some of it is likely hormones, turning on some genes, turning off others, and generally scrambling things. Some of it, too, might be a young person's search for identity. If a genetically predisposed person embarks on a quest "to be healthy" or fit or trim or a better athlete or student, it might spark that chain of events that can result in an eating disorder.

In her book Woman: An Intimate Geography, Natalie Angier writes:

As the brain stumbles toward maturity, and as it is buffeted by the output of the adrenal glands at age ten and of the gonads a year or two later, it seeks to define itself sexually and socially. The brain of a prepubertal girl is primed to absorb the definitions of womanness, of what counts and what doesn't, of what power is and how she can get it or how she will never get it.

...At the same time that hormones challenge the pubertal brain, they change the body. A girl's high estrogen content helps in the deposition of body fat on the breasts, hips, thighs, and buttocks, subcutaneously, everywhere...We can look at the deposition of body fat that comes with womanhood and say it's natural for girls to fatten up when they mature, but what natural means is subject to cultural definition, and our culture still hasn't figured out how to handle fat...we are intolerant of fatness, we are repulsed by it, and we see it as a sign of weak character and sloth. Contradictory messages assail us from all sides: we must work all the time, the world is a competitive place, and technology requires that our work be sedentary, cerebral, but we must not get too fat, because fat is unhealthy and looks self indulgent. So we must exercise and control our bodies, because our natural lives won't do it for us.

Girls, poor girls, are in the thick of our intolerance and vacillation. Girls put on body fat as they pass into adulthood. The put on fat more easily than boys do, thank you very much, Lady Estradiol. And then they are subject to the creed of total control, the idea that we can subdue and discipline our bodies if we work very very hard at it. The message of self-control is amplified by the pubescent brain, which is flailing about for the tools to control and soothe itself and to find what works, how to gather personal and sexual power. Dieting becomes a proxy for power, not simply because girls are exposed through the media to a smothering assemblage of slender, beautiful models, but because adolescent girls today are laying down a bit of fat in an era when fat is creeping up everywhere and is everywhere despised. How is a girl to know that her first blush of fatness will ever stop, when we're tearing our hair out over how the national fat index deeps on rising and we must wrestle it to the ground right now?

Go read the whole book. I've been enjoying it all weekend.