Graying hair is one of the most common and inevitable signs of aging we experience—yet it’s also one of the most mysterious. We all know someone who started graying in her early 20s while others don’t go gray until their 60s or later. So, why does hair turn gray — or even white sometimes? And why does it happen for some people sooner than others?

Well, to start, stress isn’t a factor. I know—mind blown. According to WebMD, various studies have found that, contrary to popular belief, extreme stress won’t cause your hair to go gray prematurely. And though there’s not much definitive proof of what does cause hair to turn gray, there are a few commonly believed reasons:

  • Melanin Production: The pigment in hair is called melanin, and it’s produced by cells in the hair bulb. As we age, the cells stop producing that pigment, causing our hair to grown in gray (with minimal melanin) and white (with no melanin), according to
  • Genetics: Many studies have shown that the time and extent of melanin decrease in hair is linked to genetics, and people are predisposed to go gray at or around a certain age. Also, in general, according to a WebMD, Caucasians start graying in their mid-30s, Asians in their late 30s, and African-Americans and Africans in their mid-40s. And, per a study published in the International Journal of Trichology (which focuses on the scalp and hair), graying is considered to be premature if it happens before age 20 in white people, 25 in Asian people, and 30 in African people.
  • Hydrogen Peroxide Build-Up: No, we’re not referring to the hydrogen peroxide that comes in a bottle:  Hair cells actually make small amounts of hydrogen peroxide themselves. But over time, those amounts build up and according to a study published in The FASEB Journal that may result in the growth of gray hair. Hydrogen peroxide buildup blocks the normal synthesis of melanin, thereby essentially bleaching your hair from the start. Another factor in this process is catalase, an enzyme produced by our bodies that breaks down hydrogen peroxide. But, with time, we produce less of the enzyme, which means there’s less of it to break down the hydrogen peroxide.
  • Environmental Factors: According to the study published in the International Journal of Trichology, many studies have also found a link between various environmental factors and graying hair. Things like ultra-violet light exposure, nutritional deficiencies (primarily vitamin D3 and B vitamins), and even smoking can lead to premature loss of pigment in your hair. Yet another reason to quit smoking and eat your vitamins!

READ MORE: Expert Tips for Dyeing Your Hair Gray — On Purpose