The youngest children in kindergarten are more likely to be diagnosed with attention deficit hyperactivity disorder in early grades, a study shows, an intriguing finding for parents on the fence about when to start their child in school.

The study found younger students, especially boys, are also more likely to be started on medications for ADHD and kept on the drugs longer than the oldest children. The medications are generally safe, but can have harmful side effects.

"Doctors and therapists need to factor that into their decision-making," said study co-author Dr. Anupam Jena of Harvard Medic...