As you age, it is normal for your hair to turn gray. It has to do with the inevitable cellular aging. So why is there people under 30 with gray hair premature?
Gray hair in your twenties can be due to different causes, both related to lifestyle (exposure to sunlight, tobacco ...) and genetic inheritance:
Genetics
Premature aging is almost always linked to genetics. In other words, if there is a history of premature graying in your family, it is likely that it will happen to you too. This is known as intrinsic aging, since it is not caused by external factors, but has to do only with the genes.
Tobacco
In addition to causing wrinkles and being toxic to all organs of the body, smoking can also age the follicles, as deprives the skin of oxygen and increases free radicals. Several studies have found a relationship between smoking and the development of gray hair before the age of 30.
Stress
When a person has high levels of cortisol (also known as a stress hormone), their hair can eventually turn gray. It is not a direct cause, but yes an accelerator for some people with genetic predisposition. Stress can not only make your hair turn white, it can also make it thinner.
Autoimmune diseases
It's rare that it happens, but sometimes premature graying can be caused by an autoimmune disease. Alopecia areata, for example, causes the immune system to attack hair follicles, preventing them from growing. And in some cases, when the hair grows back, it turns white.