ADHD, also called attention-deficit hyperactivity disorder, is a behavior disorder usually first diagnosed in childhood,. It is characterized by inattention, impulsivity, and, in some cases, by hyperactivity. ADHD symptoms usually occur together; however, one may occur without the others. Hyperactivity, when present, usually starts by the age of 7. Inattention or attention-deficit may not be evident, though, until a child faces the expectations of elementary school. Attention deficit disorders are sometimes not diagnosed until adulthood.