An eating disorder is when a person eats or will not eat to satisfy a psychological need rather than a physical need. It may encompass anorexia nervosa (self-starvation), bulimia (using laxatives or regurgitating in order to avoid weight gain), excessive eating, and other less common and unspecified disorders.

Eating disorders are often characterized by several things such as drastic weight gain, wearing baggy layers to hide weight loss or gain, excessive exercise, and obsession with food intake, and low self-esteem.

People can develop eating disorders at any age. However, eating disorders are most common to begin to be noticed at age 17 but are most common from ages 14-18. The development of eating disorders in adolescence is often attributed to the stress and pressure of bodies changing and growing up.

Eating disorders can occur in both men and women. It is estimated that of the 8 million Americans suffering of eating disorders, 10% are men.

Causes

Like many psychological disorders, eating disorders do not have one single stimulus that all cases are classified under.

Eating disorders can, rarely, run in a family. People with anorexia tend to have higher levels of cortisol that is linked to stress hormones.

People with eating disorders often have low self-esteem due to pressure to be "perfect" or to be what they think people want them to be. This can come from social, friend, and family pressure. The low self esteem and stress can also be from PTSD (Post-Traumatic Stress Disorder) that is a disorder caused by a stressful and traumatic event in one's life.

Treatment

Treatments for eating disorders often include therapy, self-help strategies, and other methods to help maintain someone's progress.

One of the most important things in recovery from an eating disorder is support from the person's family and friends. It can do wonders to support and uplift the person's progress and motivate 

Sources
http://www.helpguide.org/ 
http://kidshealth.org/teen/food_fitness/problems/eat_disorder.html 
http://eatingdisorders.about.com/ 
http://www.something-fishy.org/ 
http://www.web4health.info/