The simple answer is that hair follicles do not typically stop working entirely, but the biological process governing hair production fundamentally shifts as we age. This change is not an abrupt halt but a gradual, natural progression that results in hair that is shorter, finer, and less dense. The hair growth cycle becomes altered, affecting the quantity, quality, and overall appearance of the hair produced on the scalp. Understanding this biological shift requires a look into the complex sequence that governs the life of every hair strand.
Understanding the Standard Hair Growth Cycle
The process of hair production is regulated by a continuous, cyclical mechanism that occurs independently in each hair follicle. This cycle is divided into three primary phases: Anagen, Catagen, and Telogen. Anagen is the active growth period where cells in the hair bulb rapidly divide, forming the hair shaft.
For healthy scalp hair, the Anagen phase is the longest, typically lasting between two and seven years, and determines the maximum length a hair can achieve. Approximately 85 to 90 percent of the hair is in this active growth state at any given time.
Following Anagen is the Catagen phase, a brief transitional stage lasting only about two to three weeks. During this time, the hair follicle shrinks, detaches from the blood supply, and growth ceases, preparing the hair for the final stage.
The Telogen phase is a resting period lasting approximately two to four months, affecting about 10 to 15 percent of the hair. The hair remains in the follicle, but no active growth occurs. At the end of this phase, the hair is shed during the Exogen phase, allowing a new Anagen hair to begin growing in the same follicle.
How Aging Alters the Growth Phase
As a person ages, the hair follicle experiences follicular senescence, meaning the follicle’s regenerative capability slows down. This biological aging does not stop growth outright, but it significantly alters the timing of the hair cycle. The most impactful change is the progressive shortening of the Anagen, or active growth, phase over time.
The Anagen phase, which might have lasted five years in a younger adult, may only last for two or three years in an older person. This reduction means hair cannot reach the same maximum length, leading to the perception that growth is slower. Simultaneously, the resting phase, Telogen, becomes longer.
A longer Telogen phase means hair spends more time resting and less time growing before it is shed. The repeated shortening of the Anagen phase results in miniaturization, where terminal hairs are progressively replaced by thinner, shorter hairs. These fine, almost colorless hairs are called vellus hairs, and their increased presence reduces hair density and volume on the scalp.
Structural Changes in Aging Hair
Beyond changes in growth cycle timing, the physical composition and aesthetic qualities of individual hair fibers undergo significant structural alterations with age. The most recognizable change is the loss of color, or graying, resulting from a decline in the function of melanocytes, the pigment-producing cells. These cells become less numerous and less active, reducing melanin production and causing hair to turn gray or white.
The diameter of the hair shaft tends to decrease, causing individual strands to become finer and less robust. Studies suggest that hair fiber diameter may peak in the twenties or thirties and then gradually decrease. This reduction in thickness contributes to the overall thinning appearance, even if the total number of follicles remains stable.
Aging hair also exhibits changes in its internal structure, including a reduction in lipid content and a decrease in sebum production. This leads to increased dryness, a loss of natural luster, and reduced elasticity, making the hair more brittle and susceptible to breakage. The hair fiber may also become less uniform, sometimes leading to increased curvature, rougher texture, and a more wiry feel.
Primary Drivers of Age-Related Hair Change
The complex changes observed in aging hair are primarily driven by genetic programming and shifts in the body’s hormonal landscape. Genetics is a powerful determinant, dictating an individual’s predisposition to follicular aging and conditions like androgenetic alopecia, or pattern baldness. This inherited sensitivity determines which hair follicles are vulnerable to the effects of aging and hormones.
Hormonal fluctuations play a distinct role in both sexes. In men genetically predisposed to pattern hair loss, the primary driver is dihydrotestosterone (DHT), a potent derivative of testosterone. DHT binds to specific receptors in susceptible hair follicles, accelerating miniaturization by shortening the Anagen phase over many cycles.
In women, the key hormonal change occurs during perimenopause and menopause, characterized by a significant decline in estrogen levels. Estrogen is thought to have a protective effect, helping to prolong the Anagen phase and support hair density. As estrogen falls, the relative influence of androgens, including DHT, increases, contributing to a more diffuse pattern of hair thinning.