Teens are moody. there's no way around that, says Weitzman, and being able to distinguish a minor mood swing from a serious disorder takes practice. As a rule of thumb, any dark stage that persists beyond a week, affects friendship patterns or impairs performance in school "should raise a red flag for parents that something might be wrong," says Weitzman. Aside from depression, issues that often surface in adolescence include eating disorders, anxiety, stress and more serious mental illness, such as bipolar disorder, as well as experimentation with tobacco, alcohol, drugs and sex. At this age more than any other, and no matter how much teenagers resist, parental communication is important to catch early signs of a problem.
Adolescent Health, CDC