I noticed my first greys when I was 18. I've never coloured my hair, and have been white or nearly white since my mid-thirties -- just like my Mum. She had auburn hair in her youth, but I've only seen it in pictures and the hank that she had cut off, that I still have in a drawer. She never coloured her hair either, though a couple of her friends never believed that
but that's why I haven't -- I had a pretty good idea what it was going to do and when and have not been disappointed. All of her family had thick coarse dry hair, with at least some red in it, that turned early and nicely. Why mess with Mother Nature unnecessarily?
On my Dad's side, the hair is mostly thin, wispy, limp, does a kind of nondescript salt and pepper thing in the 40s or so. I don't know what I would have done if I'd gotten my hair from him.
Genetics have a great deal to do with it, so figure out which side of your family your hair takes after and guide yourself accordingly.