One potential source contributing to the misdiagnosis and inappropriate treatment of attention deficit hyperactivity disorder (ADHD) could be as simple as a child's age in relation to others in their grade. Studies indicate that children who are youngest in their grade are at greater risk of being diagnosed with, and medicated for, ADHD. The relative age effect (RAE), wherein younger people are described as less proficient relative to older members of the group, may explain these findings. This meta-analysis explored the overall effect of the RAE on ADHD in school-aged children. Twenty-six studies met eligibility criteria and 24 were included in the quantitative analysis. Results indicated that there is evidence that children who are the youngest in the class are significantly more likely to be diagnosed with ADHD and to be prescribed medication, SE = 0.09, t(8.87) = 16.0, p < 0.001. Implications of these findings on diagnostic and treatment practices for ADHD and school policy are discussed.