Objective:  Symptoms of bipolar disorder are increasingly recognized among children and adolescents, but little is known about the course of bipolar disorder among adults who experience childhood onset of symptoms. Methods:  We examined prospective outcomes during up to two years of naturalistic treatment among 3,658 adult bipolar I and II outpatients participating in a multicenter clinical effectiveness study, the Systematic Treatment Enhancement Program for Bipolar Disorder (STEP‐BD). Age at illness onset was identified retrospectively by clinician assessment at study entry. Results:  Compared to patients with onset of mood symptoms after age 18 years (n = 1,187), those with onset before age 13 years (n = 1,068) experienced earlier recurrence of mood episodes after initial remission, fewer days of euthymia, and greater impairment in functioning and quality of life over the two‐year follow‐up. Outcomes for those with onset between age 13 and 18 years (n = 1,403) were generally intermediate between these two groups. Conclusion:  Consistent with previous reports in smaller cohorts, adults with retrospectively obtained early‐onset bipolar disorder appear to be at greater risk for recurrence, chronicity of mood symptoms, and functional impairment during prospective observation.