OBJECTIVE: This report assesses whether age at onset defines a specific subgroup of major depressive disorder in 4,041 participants who entered the Sequenced Treatment Alternatives to Relieve Depression (STAR*D) study. METHOD: The study enrolled outpatients 18-75 years of age with nonpsychotic major depressive disorder from both primary care and psychiatric care practices. At study entry, participants estimated the age at which they experienced the onset of their first major depressive episode. This report divides the population into five age-at-onset groups: childhood onset (ages <12), adolescent onset (ages 12-17), early adult onset (ages 18-44), middle adult onset (ages 45-59), and late adult onset (ages > or =60). RESULTS: No group clearly stood out as distinct from the others. Rather, the authors observed an apparent gradient, with earlier ages at onset associated with never being married, more impaired social and occupational function, poorer quality of life, greater medical and psychiatric comorbidity, a more negative view of life and the self, more lifetime depressive episodes and suicide attempts, and greater symptom severity and suicidal ideation in the index episode compared to those with later ages at onset of major depressive disorder. CONCLUSIONS: Although age at onset does not define distinct depressive subgroups, earlier onset is associated with multiple indicators of greater illness burden across a wide range of indicators. Age of onset was not associated with a difference in treatment response to the initial trial of citalopram.