Is it possible for a teenager to start balding?

Unfortunately yes : But very unusual. Better see a doctor and make sure there is not an underlying hormonal or skin disorder.

Yes - with puberty: Heredity is the major cause of balding. Female-pattern baldness occurs as hair thins. Male-pattern baldness leads to bald spots & receding hairline. Other causes: Alopecia,Ringworm infection, scarring alopecia (from conditions like Lichen planus, Lupus & Sarcoidosis), hormonal changes (menopause, pregnancy, childbirth), thyroid imbalance, excess Vit. A intake, certain medications >