Why it’s important: ADHD rates have climbed over the past decades, and experts don’t know if that trend is partly due to overdiagnosis. The condition often first becomes apparent when kids enter school and their behavior is observed in relation to their peers. “ADHD can be tricky to assess. Teachers, parents, and doctors all work together to make the diagnosis,” says Dr. Castellanos. Being 6 rather than 5 can make a big difference when it comes to being able to sit quietly, so the developmentally appropriate ants-in-the-pants of a younger kid may seem like a behavioral issue. “This study suggests we need to be thoughtful about age when we’re making diagnostic and treatment decisions,” Dr. Castellanos says. But the findings shouldn’t be seen as a reason to redshirt all kids when considering whether to wait before starting school.