THE TEENAGE YEARS are a period of rapid and intense physical and emotional change. Even well-adjusted teens sometimes feel they are on an emotional roller coaster, happy and excited one minute, down the next. Because of this, it is often hard to know when a teenager has crossed the line from normal feelings of adolescent turmoil to depression.

As a woman who suffered from depression as a teen notes,

As a teenager I was moody and self-absorbed. Of course, that's common for teenagers, so my behavior was written off as normal. Unfortunately, I also had no interest in school, sports, clubs, etc. Part of it was the fog that was beginning to descend over my mind from time to time and part of it was the fear of failing in anything new.

While official estimates show...