According to the Mayo Clinic, children as young as two or three years old can show signs of ADHD, including being restless and hyperactive. However, due to so many children being naturally energetic at this age, it's difficult to make a definite diagnosis at this early stage. Once children are over the age of three they may show the three main signs if ADHD is present: hyperactivity, impulsivity, and inattention.
School-aged children may have more visible signs, including trouble completing a task before getting bored, inability to focus on just one activity, problems with following directions or processing information, and difficulty listening due to becoming distracted. But because these behaviors occur in many children to different degrees, ADHD can still be difficult for doctors to diagnose. If symptoms persist for more than six months, however, an accurate diagnosis can usually made at that time.