This study examines the manifestation of attention-deficit hyperactivity disorder (ADHD) among females of varying ages by testing the utility of particular tests to discriminate older from younger females with ADHD. A retrospective clinical chart review was conducted at a community outpatient mental health center for 75 girls from 4 to 19 years of age with a diagnosis of ADHD or subthreshold symptoms of ADHD. Signal detection methods were used to identify which variables best differentiate older (mean age = 12.06, SD = 2.61) from younger (mean age = 7.11, SD = 1.08) girls with ADHD. Girls with comorbid diagnoses of a depressive disorder and higher verbal IQ scores were more likely to be older. Overall, the findings suggest that approaches to diagnosing ADHD among females may need to be modified to include appropriate age-based criteria.