Hair turns grey mainly because the pigment-producing cells in hair follicles slow down and stop making color as you age, with genetics, stress, and health factors all influencing how early and how fast it happens. The hair itself does not “turn” grey after it grows; instead, new hairs grow in with less or no pigment, so your overall color gradually looks grey or white.

What actually changes in the hair

  • Each hair follicle has melanocytes , cells that make melanin (the pigment that gives hair its color).
  • Over time, melanocyte stem cells in the follicle are lost or stop working properly, so new hairs grow in with reduced or no melanin.
  • When many such unpigmented hairs mix with remaining pigmented hairs, the result looks grey or “salt-and-pepper.”

Main reasons your hair turns grey

  • Aging: Natural aging is the biggest factor; almost everyone loses pigment in their hair follicles with time.
  • Genetics: Family patterns heavily influence when you first notice grey; some people see strands in their 20s, others much later.
  • Oxidative stress in the follicle: Buildup of hydrogen peroxide and lower levels of protective enzymes like catalase can damage pigment production, essentially “bleaching” hair from the inside.

Role of stress and lifestyle

  • Intense or chronic stress is linked to faster greying, likely by affecting melanocyte stem cells through the body’s fight‑or‑flight (sympathetic) response.
  • Poor diet, smoking, and exposure to free radicals may increase oxidative stress in follicles, which can speed up pigment loss.
  • In rare cases where greying is tied to stress, illness, or nutrient deficiency, some small segments of hair color may partially recover if the trigger is removed.

Can greying be stopped or reversed?

  • Right now, there is no proven, widely available treatment that fully restores natural pigment once age‑related greying is established.
  • Some experimental and early research hints that managing stress and improving overall health might slow or modestly influence greying in certain people, but results are variable and not guaranteed.
  • Most people either embrace their natural grey or use hair dye, glosses, and root touch‑ups to cover or blend it cosmetically.

TL;DR: Hair turns grey because pigment cells in the follicle stop supplying color to new hairs, mainly due to age and genetics, with stress and lifestyle nudging how quickly the change shows up.