The age at onset of bipolar affective illness was determined for 100 males and 100 females who met the newly proposed DSM-III criteria for mania. Three different indices of onset were employed: first hospitalization, first treatment, and first apparent symptoms. A table was constructed indicating the cumulative percentage of those who became ill by the time they passed through each age quinquennium. One third were hospitalized before their 25th birthday, and at least 20% had already shown evidence of illness as adolescents. The early 20s was the peak period of onset. No manic episodes were confirmed prior to age 13, and onset after age 60 was rare. Sex, ethnicity, and socioeconomic status did not significantly affect age at onset. Affective illness should be given serious consideration in the differential diagnosis of mental disorders in young people.