Hello,
I understand your concern. At such young age,
male pattern baldness is not to be excluded, even though you have not noticed hair loss or hairless areas on the scalp. Hair loss in some men is hereditary and it can start showing even at 15 years old. It usually manifests with hair thinning, and not necessarily with bald patches. Looking at your family history, like male parent, siblings or relatives should give you some clues.
If this is not the case, other causes for hair thinning should be taken in consideration. Smoking and exposure to toxins in the environment and the food can be plausible causes. Stress can also be a contributing factor.
Ultimately,
autoimmune disorders, such as a low functioning thyroid gland can be major cause, although, in this case, other symptoms such as weakness,
tiredness, weight gain,
dry skin, intolerance to cold are to be factored in.
Sometimes, simple changes in life style, like switching to a healthier diet rich in
vitamins and minerals, and reducing stress, can bring about the wanted results.
Wishing you all the best.