Is it abnormal for a teenager to take medication for depression?

No. Various antidepressants are approved by the fda to be used in the adolescent popuation. It is not unusual, or inappropriate, for them to be used if the level of depression is significant and if the medication use is being well monitored.
Maybe. Many teenagers do take medication for depression, however, many of the drugs used are contraindicated for teenagers because they can increase the risk of suicide. It is more important to find the real medical cause of the depressed symptoms than just covering the symptoms with drugs.