It is thought that between 10% and 18.5% of people suffer from major depression during their adolescence. The peak age of onset is 15 with the average length of an episode of depression being about 6 months. Girls are thought to be particularly vulnerable and a team of researchers from Oregon Research Institute and the University of Texas, Austin looked into this issue by following a sample of 496 girls over 7 years, between the ages of 12 and 20. About 1 in 6 of the girls experienced major depression which lasted for an average of 5.3 months and peaked at age 16. Being white and younger were associated with greater feelings of worthlessness and wanting to kill oneself during periods of depression. 1 in 5 girls suffered from minor depression and adolescents from minority groups were more at risk from this.
Rohde, Paul ... [et al] - Major and minor depression in female adolescents: onset, course, symptom presentation and demographic associations Journal of Clinical Psychology December 2009, 65(12), 1339-1349