There’s been a lot of talk lately about how young children—especially boys—are being overdiagnosed with ADHD (attention deficit hyperactivity disorder) and overmedicated.

A new study in the journal Pediatrics adds another wrinkle. Not all kids are equally likely to be diagnosed and medicated. In fact, those in the youngest third of their class are 50 percent more likely to be prescribed a drug for ADHD than the older kids in the class.

If you’ve ever read Malcolm Gladwell’s Outliers, this situation will seem oddly familiar. In his book, Gladwell points out that a very disproportionate number of professional hockey players in Canada are born in January, February, and March. Sure, they’re talented, but they’re also among the older kids on their teams. Older kids tend to be bigger, stronger, more coordinated, and quicker, so they catch the eye of their coaches and get more attention.

Same goes for the young kids in classrooms. The youngest kids are usually going to be the least mature, the loudest, the most impulsive, and the least likely to blindly obey their teachers. They also tend to underperform on standardized math and English tests, at least relative to their peers. As a result, those same teachers figure that there must be something wrong with the kids—after all, they’re louder, more impulsive, do worse on tests, etc than their classmates—and recommend a visit to the school psychologist who, quite often prescribes Ritalin or another similar drug.

Worst of all, young kids who start taking ADHD meds often stay on them through their school years.

Sonia Hernández-Diaz, an associate professor of epidemiology at the Harvard School of Public Health and coauthor of the study, said “Age should be considered when evaluating children for an ADHD diagnosis and a prescription of a stimulant such as Ritalin.”