I have a couple of gray hairs. At the age of 43, I have much less gray hairs than other people my age but I see they are coming. A popular claim is that gray hair is caused by stress, which is odd because I almost never feel any stress. Well, it turns out that there are numerous studies demonstrating that stress does not cause graying hair. Gray hair is actually correlated with age and is mostly genetics. The primary cause is that the melanocytes (the color producing cells) somehow get turned off. For the latest info on research in this area, check out this story about gray hair in the New York Times.