I found my first gray/white hair the other day. It was right in the very front, next to my forehead. I regularly check for grays because my older sister started getting a few gray hairs at my age, and so did my mom. I know it is a perfectly normal sign of aging, but it freaked me out just a little bit (which means a lotta bit ;)
As a hairstylist I deal with gray hair almost daily. But it was a little different to find it on my own head. When someone goes gray, it is largely due to genetics (strike one), but other things in life definitely play a factor.
Autoimmune diseases can cause premature graying (strike two)
Extreme stress in your life can start the graying process (strike three)
I had taken a break from coloring my hair (okay, okay, a break for me is about 4 months) but apparently I'm going to have to start up again soon. Good thing I can do it myself, because the cost of vanity can be very pricey ;)