Objective: This study examines the relationship between age at onset of bipolar I disorder and illness characteristics among adults in a community sample. Method: The National Epidemiologic Survey on Alcohol and Related Conditions identified 1,411 adults with bipolar disorder. For analyses, bipolar disorder subjects were divided into three age at onset groups: childhood (less than 13 years old, N=113), adolescence (13–18 years old, N=339), and adulthood (19 years or older, N=959). Results: Nonremitting bipolar disorder was most prevalent among childhood-onset subjects, and childhood-onset subjects were most likely to experience prolonged episodes. Antisocial personality disorder was most prevalent among childhood-onset subjects. Drug use disorders were more prevalent among childhood-onset and adolescent-onset, as compared with adult-onset, subjects. Prevalence of mixed episodes or irritability did not differ significantly between groups. Conclusions: Findings corroborate clinical studies: illness characteristics among adults with childhood-onset bipolar disorder are similar to those described in children with bipolar disorder.