Human hair color is a biological trait that is not permanently fixed from birth. Many people experience a noticeable shift in their shade, particularly during the early years of life. This timeline of color evolution is governed by genetics and developmental changes, leading to distinct phases before the final adult shade is established.
The Role of Melanin and Genetics
The color of a person’s hair is determined by melanin, a pigment produced by specialized cells called melanocytes within the hair follicles. There are two primary types: eumelanin and pheomelanin. Eumelanin is responsible for darker shades (brown and black), where a higher concentration leads to a darker color. Conversely, pheomelanin provides red and yellow hues, dominant in light hair colors and redheads. All human hair contains a combination of both pigments, and their precise ratio dictates the final shade. Genetic instructions determine the activity of the melanocytes and the balance of these pigments produced throughout the hair-growth cycle.
The Initial Transition: Hair Color Changes in Infancy
The first color change often occurs in the months immediately following birth, as the fine, initial hair is replaced by more mature strands. Many babies are born with a dark shade that is quickly shed around three to six months of age, with the replacement hair sometimes growing back much lighter. This early shift is a result of the melanocytes in the follicle beginning to mature and fully express the genetic blueprint for color.
After the initial replacement, hair color continues to change throughout the toddler and preschool years, typically between six months and five years old. This period frequently sees a progression from lighter shades to darker ones, such as blonde hair transitioning to light brown. External influences also play a role; increased sun exposure can activate melanocytes to produce more pigment, or alternatively, cause an oxidative bleaching effect that temporarily lightens the hair.
One detailed study tracking children found that hair was often darker in the first half-year of life, then lightened between nine months and two and a half years, before beginning a progressive darkening trend until age five. These shifts highlight that the hair color seen in a young child is often a temporary, developing shade. The thickness of the hair strand also increases significantly during this period, which contributes to a visual change in the perceived depth of color.
Stabilization: When the True Adult Color Emerges
The final developmental change in hair color occurs as a child enters and progresses through puberty. Hair color stabilizes into the adult shade around late childhood or early adolescence, typically between the ages of eight and fourteen. This stabilization is directly related to the hormonal shifts and developmental growth plateaus of the body.
The genetically programmed level of melanin production is fully activated and locked in during this phase, often resulting in a final shade that is darker than the childhood color. The influence of sex hormones, which surge during puberty, is responsible for this final maturation of the hair follicle’s pigment production. For many individuals, this means a transition from light blonde or golden shades to darker browns or even black hair.
Once the color has stabilized in the early teen years, the hair’s natural shade will not undergo another fundamental, genetically-driven change in its base color. The hair follicle has reached its mature, genetically determined output for melanin. Any changes observed after this point are attributed to entirely different biological or environmental mechanisms, rather than the developmental shifts of childhood.
Later Shifts: Graying and Environmental Factors
Following the stabilization of the adult color, any subsequent changes are a sign of aging or external damage, not a continuation of the developmental process. The most well-known later shift is achromotrichia, commonly referred to as graying, which is a process of pigment loss. This occurs when the melanocytes in the hair follicle gradually stop producing melanin, leading to a completely colorless hair shaft.
The onset of graying is largely determined by genetics, with the average age for the first noticeable gray hairs appearing around the mid-thirties, though this varies significantly between individuals and ethnic groups. Since the hair is no longer pigmented, it appears gray or white.
External factors can also cause temporary or superficial changes to the adult hair color. Exposure to ultraviolet radiation from the sun can break down the melanin in the hair shaft, resulting in a lightening or bleaching effect. Similarly, chemicals like chlorine found in swimming pools can interact with the hair shaft, often causing a greenish tint on lighter shades.