Yup, it's definintely not just you! I just turned 19 and recently decided to try and grow out my natural colour, since I've been colouring my hair since I was 16. Well I have almost an inch of roots right now, and last week I was just checking them out in the mirror, and I found a white hair! It was only white the top inch [obviously, since the rest is dyed]... but now it makes me wonder just how many more are hiding there, LOL.
My mother started going grey when she was 16!! But her life was full of stress... I don't think it's genetic, because her mother barely had any grey, even when she was 40-something. My mom's 45 now and probably 75-80% grey, no kidding [she just started colouring it last year] . I really hope I don't end up with her luck, haha.
My boyfriend is turning 21 and he has quite a few grey hairs. When I look at his hair I can see maybe a couple dozen here and there. But he has sooo much hair, it's so thick and curly, and really really dark brown, that you don't notice the greys unless you're really looking.