Objective: Research about Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV) age of onset criterion for ADHD led to increasing that criterion to 12 in Diagnostic and Statistical Manual of Mental Disorders (5th ed.; DSM-5). Although an improvement from the previous age at onset of 7, whether this new criterion is adequate to capture all cases with ADHD remains uncertain. Method: We compared three groups of adults: (a) ADHD participants who met all DSM-5 criteria for ADHD (n = 182), (b) late-onset ADHD participants who met all criteria except for later age at onset (n = 17), and (c) non-ADHD participants who did not meet criteria for ADHD (n = 117). We assessed patterns of symptoms, psychiatric comorbidity, functional impairment, familial transmission, quality of life, social adjustment, and intelligence. Results: Compared with non-ADHD participants, all ADHD groups had poorer quality of life and had more impaired social adjustment. Compared with each other, the ADHD groups had similar patterns of psychiatric comorbidity, functional impairment, familial transmission, and intelligence. Conclusion: When assessing adult ADHD, self-reported onsets of ADHD after 12 years of age may be warranted.