To add to the previous answerer, if you look at others closely, you might be surprised to notice that many people in their late teens and early 20s already have a few grey hairs. It isn't uncommon at all. Also, just because you have a couple here and there doesn't mean that you will soon have a head of grey hair.
Its just that the pigmentation cells that produce the color for your hair stop doing so. Grey hair is primarily genetic and due to heredity, although some can result from certain vitamin deficiencies. Some research suggests that stress can bring on grey hairs as well.White dots on testicals?
You have a rare disease, in which every year you would age is worth 15 years. Sucks for you.
Hasta la vista, granny.