Why does hair start growing in gray when you get older?

I’m only 21, but I’ve noticed that I’m starting to get in some grays already. My hair is a dark red/brown normally and mom had dark brown hair, too… Now her hair is all blackish and silver.

Anyway, can any of you come up with a scientific answer for this.. or maybe even a philosophical answer for why does hair grow in gray when you get older? I think it’s an interesting topic.