My mom said I had them since birth, but I didn't notice my few grey strands until about 12 years old. Now I have a racing stripe in the middle front of my hairline. So far I haven't noticed them anywhere else, however looking at rest of paternal relatives, I could wake up tomorrow with a head full of white hair.
I still have not decided what I am going to do when that happens. I hate the idea of having to maintain color, but I if the grey ages me considerably then I may color. Maybe I'll do the henna/indigo thing. I henna every once and a while now, which does change the color of my streak, but I don't maintain it (I have about a good inch and half of grey attached to my orange/gold streak, it doesn't bother me one bit.