The research: Many states have a cutoff birthdate of September 1 for kids to enter kindergarten. That means those who were born in August, right before the cutoff, can be nearly a year younger than classmates with a September birthday. A study in the New England Journal of Medicine using insurance data from more than 400,000 kids found that August-birthday kids in these states were diagnosed and treated for ADHD at higher rates than their peers born in September.