I have a a four-year-old nephew, Landon, who is exhibiting traits of obsessive-compulsive disorder (OCD). He insists the blocks be arranged by color and cries when they are not. Autism has been ruled out, so the next suspect is OCD. But is it really OCD, or is it just a phase? How young is too young to make a diagnosis of mental illness? And when a diagnosis is made, at what age should treatment start?