Objective: To provide additional information about clinical features associated with adult ADHD in patients diagnosed in childhood compared to those first diagnosed in adulthood. Method: We stratified a sample of adults with ADHD into patients diagnosed in childhood versus adulthood and compared demographic and clinical characteristics. Results: We found similar clinical features in adults diagnosed in childhood and adults diagnosed in adulthood. Among those diagnosed in adulthood, 95% reported symptom onset in youth. Our results do not support the hypothesis that ADHD diagnosed in adulthood is due to misinterpreting symptoms of other disorders as ADHD. They also suggest incorporating behavioral signs of executive dysfunction into diagnostic criteria for ADHD in adults may increase diagnostic sensitivity. Conclusion: These results support the validity of ADHD diagnoses in adulthood, as these adults show similar clinical profiles to those diagnosed in youth. Our results also suggest that if adult-onset ADHD exists, it is rare.