The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM‐5), requires that symptoms of ADHD must be developmentally inappropriate in order for an ADHD diagnosis to be considered. Because the DSM‐5 does not specifically outline procedure for determining developmental inappropriateness of behaviors, practitioners do not have guidance for making this consideration. To explore the relationship between mental age and behavior ratings of ADHD symptoms, the cognitive and behavioral data from 50 children who were evaluated in a school or clinic were compiled for this study. Cognitive ability scores were used to calculate mental age and behavior rating scores were then recalculated using the assessed mental age. Using the recalculated behavior ratings, scores based on mental age decreased significantly on the Attention Problems, Hyperactivity, and Atypicality scales of the BASC‐2. Additionally, as the discrepancy between mental and chronological age increased, larger differences in scores were found.