This report explores the relationship between age of first onset of major depression and other demographic and clinical features in the first 1500 patients entering the Sequenced Treatment Alternative to Relieving Depression (STAR*D) study. Outpatients, 18–75 years of age, with nonpsychotic major depressive disorder (MDD) from either primary care or psychiatric practices constitute the population. Age of onset was defined at study intake by asking patients to estimate the age at which they experienced the onset of their first major depressive episode. This report divides the population in terms of pre-adult (before age 18) onset and adult (age 18 or later) onset. The results suggest that MDD that begins before age 18 has a distinct set of demographic (female gender) and clinical correlates (longer duration of illness; longer current episodes; more episodes; more suicidality; greater symptom severity; more psychiatric symptoms associated with Axis I comorbidity; and more sadness, irritability, agitation and atypical symptom features), and it appears associated with significant psychosocial consequences (lower educational attainment and marriage rates). Thus, pre-adulthood onset MDD is a particularly severe and chronic condition.