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.

“Doctors and therapists need to factor that into their decision-making,” said study co-author Dr. Anupam Jena of Harvard Medical School. They should ask, “Does he really have ADHD, or is it because he needs six more months to mature? That extra year makes a big difference.”

About 6 million U.S. children and teenagers have been diagnosed with ADHD, which causes inattention, hyperactivity and impulsivity. The rate of is climbing.

The study, published Wednesday by the New England Journal of Medicine, stemmed from a lunchroom conversation about “kindergarten redshirting” for a co-author’s son. The term is borrowed from athletics and means waiting a year to give a child time to mature.

“The parents were thinking about whether or not to hold their child back an additional year,” Jena said. That led the researchers to ask, “What happens to kids who are in the same class who are perceived to be different?”

They used insurance claims to compare more than 71,000 students with August and September birthdays in 18 states with Sept. 1 cutoffs. A child who turns 5 before Sept. 1 can start kindergarten. If not, the child waits until the next year. An August birthday can mean a child is the youngest in class while those born in September are the oldest.

The researchers found the rate of ADHD diagnosis was a third higher in August-born kids than in September-born kids.