My hair starting turning white when I was in 7th grade, I had hair practically down to my waist. The white started in the back, one of my friends that sat behind me pulled one out. As time went on more and more came in, underneath at first and then slowly more on the top. By the time I was 31 I was salt and pepper so I cut it short. I dyed my hair until I was about 39 and by then my hair was so white and it grows so fast that my roots were showing in no time, so I stopped coloring it. For the past ten years my hair has been white. I consider myself very lucky, because it is very shiny and healthy looking. I do not think it makes me look older that I actually am.
My family all started to go white early, but they started in their 20's not their teens like me. At one point my hair was actually whiter than my moms because she was just letting her grey stay around her temples, etc.
It does seem though once you have children it does go whiter faster, it that because they drive you crazy or you just are lacking something in your vitamins?