 In contrast to these patterns, prevalence of clinical depression is high in all age categories, including otherwise healthy adolescents and young adults.In one study of the US population, for example, the 12 month prevalence for a major depression episode was highest in the youngest age category (15- to 24-year-olds).The high prevalence of depression is also an outlier when compared to the prevalence of major mental retardation, autism, and schizophrenia, all with prevalence rates about one tenth that of depression, or less.