According to the National Institute of Mental Health, 11% of adolescents will be diagnosed with a depressive disorder by the time they reach 18 years of age.
I came across this Ted Talk that looks at this topic from the perspective of a teen who has dealt with depression throughout his life. He describes what it is and isn't, and what it feels like to him. The statement that struck a chord with me, though, was when he speaks to the fact that as a society, we would treat physical illness in children and teens with the utmost sense of urgency, not resting until we had made the kid well. However, with mental illness like depression, we can have a tendency to blame rather than support the person afflicted.
For more information, watch the full video below: