It can be diagnosed as early as age 3.
Preschoolers naturally have a short attention span, so doctors may be hesitant to label them with ADHD. Kids are typically diagnosed around age 6 -- when they're expected to sit quietly in school, follow directions, and not interrupt -- but the disorder doesn't suddenly appear then. "ADHD is a chronic condition that can start early," says Parents advisor Harold Koplewicz, MD, President of the Child Mind Institute. Signs include aggressiveness (which often gets children kicked out of childcare or preschool), trouble focusing on activities like bedtime stories, and uncontrollable behaviors such as bolting into a busy parking lot. "By the time they're diagnosed with ADHD, most kids have had a long history of problem behaviors that can affect their self-esteem," says Dr. Koplewicz. In fact, a large study from the National Institute of Mental Health (NIMH) showed that children ages 3 to 5 can benefit greatly from treatment. Researchers found that low doses of medication are safe and effective, but since preschoolers are more sensitive to side effects than older kids are, they need to be closely monitored.