I started getting grey hairs at a very young age, around 6 or 7. I'm now 30 and have a good amount of scattered greys. I've never had a problem with it and my wife thinks it's sexy.
I've always attributed to stress as others have said and my mother swears it started happening after my brother broke his arm the first time. I was right there when it happened and had to run home to get my dad. Also, no matter what he says he fell off that jungle gym by his damn self, I had nothing to do with it...just wanted to point that out.