Each hair on a person's head grows for between two to six years, rests for three months and then sheds, according to Health Scout Network. (Ref. 1.) When a hair is shed, it is replaced by another hair from the hair follicle. Teenagers experience hair loss if they shed more than 100 hairs each day. (Ref. 1.) Although hair loss is common for middle-aged and elderly adults, teens can lose hair, often due to medical conditions.

Alopecia Areata

Alopecia areata is a skin disease that causes teens to lose hair on their scalp and sometimes on other parts of their body. According to MedicineNet, the condition is caused by an irregular immune system. (Ref. 2.)

Traction Alopecia

A form of baldness called traction alopecia can be caused when teens regularly curl or bleach their hair or pull it into tight hairstyles, according to All4naturalhealth.com. (Ref. 3.)

Other Diseases

Diseases that affect the endocrine, digestive and urinary systems, such as diabetes, thyroid disease or chronic kidney disease, can cause unexplained hair loss in teens.

Trichotillomania

Trichotillomania is a psychological condition that causes people to pull out pieces of their hair, leaving bald spots.

Malnutrition

People need to eat a regular amount of protein, found in meat, fish, eggs, grain products and nuts, to stimulate hair growth. Teens who don't eat protein regularly, especially vegetarians, can become malnourished and experience hair loss.

Chemotherapy and Medications

Drugs used in chemotherapy kill fast-growing cells in teens' bodies, including hair cells. Hair generally falls out between two and three weeks after the start of treatment, according to the National Cancer Institute. (Ref. 4.) According to the Nemours Foundation, other drugs, such as lithium, isotretinoin and diet pills, can also cause hair loss. (Ref. 5.)

Source:

Health Scout Network: Hair Loss

MedicineNet, "Alopecia Areata," Zoe Draelos.

All4naturalhealth.com: Hair Loss in Teens-Possible Causes