Schilder's disease is a rare condition that usually starts in childhood. It's most common in boys between 7 and 12 years old. It's a problem with the body's myelin. Myelin is a protective layer that covers most of the nerves in your body. It's like the coating on electric wires. It helps signals move faster around your body, the way electricity flows from a power source. When the myelin is damaged, signals can't move the way they should.