Grey hair can show up in the 20s, and for some people it can begin even in the late teens. That does not automatically mean something is wrong. Most people will start to see gray hair in their mid-30s, but the timing varies widely, and by age 50, half of all men and women will have at least 50 percent gray hair.

Mona Gohara, a dermatologist, said gray hair in the 20s can be influenced by genetics, meaning that if parents or grandparents grayed early, a person is more likely to do the same. She also said melanin production in hair follicles decreases over time, and that when there is less melanin, hair loses pigment and turns gray. White hair reflects a total lack of pigment.

That biological change helps explain why gray hair is so common with age, but it does not explain every case. Oxidative stress, lifestyle factors and some health conditions can speed up graying. Gohara said vitamin B-12 deficiency, thyroid issues or pituitary problems can lead to premature graying, though those causes are less common than genetics.

For people worried about early gray hair, a study of premature graying in people under 20 found lower copper levels appeared in those affected. If low copper is present, a diet rich in shellfish, seeds, nuts and fish may help. The point is not that one food will reverse the process, but that in some cases a nutritional gap may be part of the picture.

The larger answer is straightforward: there is no normal time to go gray. Some people notice the first strands in their 20s, others not until their 30s, and many see a much bigger shift only later in life. Early graying is often genetic, and when it is not, the medical causes are real but comparatively uncommon.

So if a 24-year-old spots a few silver strands, it is usually less a warning sign than a family trait arriving early. The next step, if the change is sudden or accompanied by other symptoms, is to look for an underlying cause rather than assume gray hair itself is the problem.