Why would a 15-year-old have a small bald spot on the left side of his head that has been there for a long time?

If the bald spot started out the size of a dime, and is growing, it could be alopecia areata, A dermatologist can give you a shot of cortizone to step the growth up or without treatment, it will grow back within a year. Stress is usally the cause of this. I have seen 4 cases of this.