Gray hair occurs when the hair follicles produce less melanin, the pigment responsible for hair color. As people age, the number of melanocytes, the cells that create melanin, decreases, leading to a loss of color in the hair. This process can begin in a person's 20s or 30s, but the timing varies widely among individuals. In addition to aging, factors such as genetics, stress, and certain medical conditions can influence the onset of gray hair. Some people may experience premature graying due to hereditary factors, while others may notice changes in hair color due to lifestyle or health issues.