ADHD or attention deficit hyperactivity disorder is a mental disorder that makes it hard for a person to pay attention and control their impulsive behavior. People with this condition also have difficultly in sitting still. It is a chronic condition that affects millions of children. Though it generally starts at a very young age, as children as young as three years of age begin to show the symptoms of ADHD, it is not exclusive to children. Many adults also have this condition, though the severity can differ significantly between younger and older people.