Some people just get it early. I know some teens get a few white hairs because there stressed. Maybe the person or child is really stressed about something. Or maybe its just something there going to have at an early age. Remember Taylor Hicks? He had gray hair when he was only 15. Nothing to be worried about.
It's when the hair looses all pigmentation making it white. It can happen only in patches or it can make all of their hair white. Generally it's an inherited condition but it can be caused by other things.