According to the American Academy of Pediatrics, ADHD can be diagnosed in children as young as four years old. However, it is difficult to diagnose ADHD in children younger than four years old because younger children change very rapidly. To be diagnosed between the ages of four and 16, a child must show six or more symptoms for more than six months, with most signs appearing before age 12. Symptoms must be present in two or more settings and interfere with the quality of social or school functioning. It is important to note that ADHD symptoms can change over time as a child grows and moves into the preteen and teenage years.