I noticed 1 or 2 grey hairs when I was about 20, by the time I was 24 I was dying my hair. I'm 31 now and still having to colour my hair. My brother has gone really grey and he is only a couple of years older than me. DH is blond so you can't really see his grey.
I don't know if you have noticed that people these days look younger, it is becasue of hair dye. Just have a look at some old photos.