I know that some of these behaviors seem to be normal for preteens and teens, but it is the change in behavior that is the key. If your preteen used to spend a lot of time with friends, but all the sudden stays in their room, or usually loves a family outing and now refused to attend, or starts becoming very secretive - these are changes in behavior and you need to seek help from a doctor or mental health provider.
Early diagnosis and treatment are essential for depressed children. Depression is a real illness that requires professional help. Comprehensive treatment often includes both individual and family therapy, as well as the use of antidepressant medication. For help, parents should ask their physician to refer them to a qualified mental health professional, who can diagnose and treat depression in children and teenagers.