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Although most people typically associate white hair with getting older, there is no specific age at which people are "supposed" to go grey.  Some people spot their first white hairs in their teens, while others don't go grey until middle age. As the presence of white hair is mainly down to genetics, if your parents went grey at an early age, you probably will too. Race also plays a role. Most white people will start to see white hairs by the age of 35, Asians by the age of 40 and black people by their mid-forties. It's a common belief that white hairs are caused by stress, but there is no clear scientific proof for this theory. It's possible that stress can contribute to the production of free radicals that result in gray hair, or that it may prevent melanocytes (stem cells involved in creating new hair) from delivering pigment to your hair. While there is some limited evidence backing these possibilities, and while many doctors believe stress can hasten graying, this has not been proven conclusively.  Hair turns white when the color-producing cells in the hair root stop producing pigment, such as Melanin (which is what gives hair its color). Genetics may outline when and how this process happens in each person. It has also been suggested that a build up of hydrogen peroxide around the hair follicles can bleach color from the hair as a result of oxidative stress. However, stress has many other negative effects on your health (including possible hair loss) so reducing stress is still a good idea. Sometimes when a person's hair turns white prematurely, it's the result of an underlying autoimmune or genetic condition.  Some specific conditions that are sometimes linked with hair turning white include vitiligo (a thyroid disorder) and anemia. Problems with the pituitary gland can also be a factor. Therefore, if you're hair is turning white prematurely and you experience other symptoms attached to any of these conditions, you should see your doctor.
Know that white hairs are mainly due to genetics. Understand that there is no proven link between stress and white hair. Check for any potential underlying conditions.