In men, the most common cause of long term, permanent hair loss is a condition known as male pattern baldness. This is a genetically inherited condition that results in different patterns of hair follicles that are sensitive to the male hormone DHT.
Only the hair follicles sensitive to DHT are affected, and the effect is to shorten the active growing phase of the hair cycle, thus causing them to shrink. Eventually the hair cycle stops and the hair never re-grows.
The effect of DHT on these sensitive hair follicles often varies from person to person. Effects typically begin at puberty when both testosterone and DHT begin to circulate in men. But because an area of the scalp does not appear thin or balding until one loses about 50% of the hair in that area, most patients do not notice the change until they are in their late 20's or 30's. Occasionally a person will complete the hair loss process before age 25.