False. 
Have you ever heard someone say something like “you are going to give me gray hair” or “all this stress is going to turn me gray”? I actually remember saying this a few times in my life, for example when our children were progressing through the terrible twos. Well, there really is no scientific evidence that links stress and hair turning gray. Graying often starts in the mid 30’s for Caucasians and the mid 40’s in people of color. However, graying can start as early as the mid to late teens or not start until an individual is in their 50’s or 60’s. If you read much on this topic you will likely come across the 50/50/50 principle. That is roughly 50% of people will have 50% of their hair turn grey by the time they are 50 years old. As I sit and write this chapter I happen to be attending a meeting and am sitting directly behind a woman in her mid 50’s. She has very long pretty hair, and I’d say that somewhere between 50% and 60% of her hair is grey. I’m currently 40 years old and started noticing the appearance of grey hairs 5 or 6 years ago and it is progressing quickly! My father is in his mid 70’s and has brilliantly white hair, so I anticipate in the next 5 to 10 years I will be completely gray. Some people work hard to cover up their gray hair with things like artificial coloring products, where others just accept that graying is a normal part of the aging process. Some people even like to see their hair turn gray as they think it makes them look distinguished. What actually causes gray hair? Hair has the color it does due to pigment, this pigment is called melanin. The cells that create melanin are called melanocytes. As we age, melanocytes die or produce less pigment resulting in gray hair. An article (Trueb 2005) about aging hair also states that genetics play a role and autoimmune disorders can turn hair gray.
Reference:
Trueb, R: Aging of Hair. Journal of Cosmetic Dermatology (2005), Vol 4, pps. 60-72.