According to KidsHealth, there are multiple reasons for hair loss, or alopecia, in teenagers. Some of the causes include androgenic alopecia, alopecia areata, various medical conditions and taking certain medications.
Some teenagers lose hair due to self-inflicted causes, including a psychological condition called trichotillomania, the sufferers of which pull out their own hair. According to WebMD, poor nutrition is another common cause for hair loss. Poor nutrition can include not eating enough, suffering from an eating disorder and eating too many unhealthy foods. If the hair loss is from a condition called telogen effluvium, it is temporary, and teens can expect their hair to return.