A close friend of mine in college had gorgeous dark brown hair, and he always had a few stray white hairs. He said it was because of stress, but can that really affect your hair?
Hair is a protein that gets its color from a pigment known as melanin - the same pigment that gives your skin its color. Over time, the amount of melanin in your hair decreases. Gray hair is simply hair with low levels of melanin and white hair has no melanin at all.
So can fear or stress change your hair color? It sounds believable to think that your emotional state can have a significant impact on the hormones that affect the amount of melanin deposited in each strand of hair, but there is NO scientific proof that this is true.
Most men start to turn gray in their early 30s, and for most women, it starts to happen after 35. Genetics can also play a role - causing you to develop premature white hair in your 20s. There's no way to prevent the graying process from happening - it's just a wonderful part of growing old.
While we're on the subject - if you do find a gray or white hair, and you pluck it out, contrary to popular belief, two more gray hairs will NOT grow in its place, so feel free to pluck away!