Some people get a few white/gray hairs very young, but they don't turn into much. One possible reason for premature graying can be a vitamin deficiency. There are other causes, but making sure you get the right combination and amounts of vitamins, minerals, etc., is probably one thing to do.
I got a few gray hairs coming out of a scar I've had for decades. At the time I was in my late thirties, and I was worried I was turning prematurely gray. The gray only came from the scar (follicle damage is another possible cause for early graying, so I'm assuming follicles in scar tissue may be more likely to "have issues").
In my forties I saw a few more, but even then I could "pass them off as blond highlights" because I have light hair. For the most part, even with an occasional one showing up, my hair has remained its original color (although these days, well past my fiftieth birthday) I'm starting to notice that my light brown hair seems to be looking "blonder". The days of not needing to color my hair seem to be numbered at this point.
The point is (and everyone is different), even more than ten years after seeing those first few, hard-to-see, gray hairs; my hair is still brown (with a few "highlights" here and there. :) ).
Here's a pretty good page that may be reassuring to you, or anyone, with a few gray hairs. It also offers some other things to keep in mind if you believe your hair is prematurely (and "seriously") graying:
Apparently, based on information on that page, 50% of people start to go gray before 50 years old.