Hair growth does not stop at a certain age, but the process undergoes significant and noticeable changes as the body ages. Hair is a biologically continuous process, with individual strands cycling through periods of growth, transition, and rest throughout a person’s lifetime. The perceived cessation of growth or the development of thinning and graying is actually the result of natural shifts in the hair follicle’s function. These age-related alterations are driven by a combination of genetics, cumulative cellular damage, and hormonal fluctuations that directly impact the hair’s growth cycle and its pigment-producing cells. The changes lead to shorter, finer hairs and a loss of color, altering the overall density and appearance of the hair over time.
Understanding the Hair Growth Cycle
Hair growth operates in a continuous, multi-stage cycle that dictates the length and thickness of each individual strand. This cycle is divided into three primary phases: anagen, catagen, and telogen.
The anagen phase is the active growth period, where cells in the hair root divide rapidly, pushing the hair shaft out of the follicle. The duration of the anagen phase largely determines the maximum length a person’s hair can achieve, typically lasting between two to seven years for scalp hair. At any given time, approximately 85% to 90% of the hairs on the scalp are in this active growth stage.
The catagen phase follows, representing a short transitional period that signals the end of active growth. During the catagen phase, which lasts about two to three weeks, the hair follicle shrinks and detaches from the nourishing blood supply. This phase affects only a small percentage of hairs, around one to three percent, and prepares the hair for the next stage.
The final stage is the telogen phase, a resting period that typically lasts around three months. The hair remains anchored in the follicle during telogen until it is shed, making way for a new anagen hair to begin growing, restarting the entire process.
How Aging Affects the Growth Phase
The most significant age-related change is a progressive alteration in the duration of the hair growth cycle, particularly the active anagen phase. As a person gets older, the anagen phase gradually shortens, meaning hair spends less time actively growing. This reduction limits the maximum length hair can reach and contributes to a slower overall growth rate.
Simultaneously, the telogen, or resting, phase lengthens, causing hair to remain in the follicle for a longer period without growing. This shift means that a greater percentage of scalp hairs are in the non-growing stage at any one time, directly leading to a perceived reduction in hair volume.
The cumulative effect of these cycle changes is a process called follicular miniaturization, which is central to age-related thinning. Follicular miniaturization involves the gradual shrinking of the hair follicle with each successive cycle. The follicle produces a progressively thinner, finer, and shorter hair shaft until it may stop producing cosmetically significant hair altogether.
Hormonal shifts, such as the decline in protective estrogens after menopause or the influence of dihydrotestosterone (DHT) in aging men, accelerate this miniaturization process. Furthermore, accumulated cellular damage and stem cell aging within the follicle contribute to its reduced ability to maintain robust, long-term growth.
Age-Related Changes in Density and Pigment
The biological mechanisms of aging manifest as visible changes in the hair’s density and its color. Density loss, or overall thinning, occurs because the shortened anagen phase and the miniaturization process produce hairs with a smaller diameter. Fewer follicles enter the active growth phase, and the ones that do produce weaker strands, resulting in a noticeable decrease in hair volume.
The hair also becomes less pigmented, leading to graying, a process known as achromotrichia. Hair color is determined by melanin, a pigment produced by specialized cells called melanocytes located within the hair follicle.
As a person ages, the melanocyte stem cells that replenish the pigment-producing melanocytes begin to decline in number and function. When the hair follicle enters a new growth cycle, the supply of melanin-producing cells is diminished, causing the new hair to grow without pigment. This results in the hair appearing white or gray, which typically starts at the temples and progresses across the scalp. The texture of the hair can also change, often becoming drier and more brittle due to decreased production of natural oils and changes in the hair shaft’s protein structure.