Androgenetic Alopecia (AGA), commonly known as pattern baldness, is the most frequent cause of hair thinning. This condition represents a gradual, genetically-predetermined process where hair follicles progressively shrink over time. It affects a majority of the population, though the age at which it becomes noticeable and its eventual severity varies widely.
Statistical Timeline of Hair Loss Onset
The process of pattern hair loss can begin earlier than most people realize, often starting subtly after puberty. For men, early-onset AGA is already present in a significant portion of the young adult population. Approximately 16% of men between the ages of 18 and 29 show at least a moderate level of hair loss.
The prevalence of hair loss increases sharply with each subsequent decade of life. By age 50, about 50% of men experience some degree of male pattern baldness. This indicates that for many men, initial signs of thinning appear during their late twenties or thirties and progress steadily.
The timeline for women tends to be slightly delayed, although female pattern hair loss (FPHL) is also common. The mean age for the onset of AGA in women is around 29.5 years. While it may begin earlier, FPHL becomes particularly noticeable and prevalent following menopause. By the time women reach 65 years of age, up to 75% may be affected by some degree of hair thinning. The age of onset and rate of progression are highly individualized and strongly tied to hormonal changes associated with aging.
The Hormonal and Genetic Basis
The underlying cause of pattern baldness is a combination of inherited genetic sensitivity and the presence of specific androgen hormones. The primary hormonal factor is Dihydrotestosterone (DHT), a potent derivative of testosterone. DHT is formed when the enzyme 5-alpha reductase converts testosterone into this more powerful androgen.
In individuals genetically predisposed to AGA, hair follicles possess androgen receptors sensitive to DHT. When DHT binds to these receptors, it initiates follicular miniaturization. This causes the hair follicle to shrink, shortening the anagen, or active growth phase, of the hair cycle.
Over successive cycles, the hair produced by the miniaturized follicle becomes progressively finer, shorter, and lighter in color. Eventually, the follicle may stop producing visible hair altogether. The degree of this sensitivity is a polygenic trait, meaning it is determined by multiple genes.
Male Versus Female Pattern Hair Loss
Although the fundamental mechanism involving DHT and genetic sensitivity is shared, the visual manifestation of hair loss differs significantly between men and women. Male Pattern Hair Loss (MPHL) typically follows a predictable, concentrated pattern. It usually begins with a receding hairline at the temples or thinning on the crown of the scalp.
This progression is standardized by the Hamilton-Norwood scale, which classifies male hair loss into seven stages. In advanced cases, the result is often a horseshoe-shaped band of permanent hair around the sides and back of the head, where follicles are not susceptible to DHT.
Female Pattern Hair Loss (FPHL) rarely results in total baldness and follows a diffuse pattern. Women typically experience a general reduction in hair density across the top of the scalp. The frontal hairline is often preserved, but the central part line tends to widen.
The Ludwig scale is used to classify the severity of FPHL, focusing on the degree of thinning on the crown. Because women generally have lower levels of androgens than men, the hair loss progression is often slower and presents as generalized thinning rather than the defined balding areas seen in MPHL.