My mom was 80 when she passed away and I don't think she was quite 50-50 yet. My hair is quite similar to hers (wavy/curly, dark brown, and started thick and coarse and is thinning and getting finer with age). I feel like I have more grey at my age than she did, but that may not be the case -- when I look at photos of her in her 50s, she clearly had some grey, but the overall impression of her hair was still definitely a brunette. My dad died at 65 and had some grey, but not much.
So it's highly unlikely I will ever go completely grey, maybe unless I live to be 100 (which I am aiming for!).
I know some people -- one of my friends who is of Greek descent for one -- who seem to have no grey at all even though they are approaching 60 and have never colored their hair. As someone who is not going grey prettily, I envy them!
PS. I think I saw my first grey hair in my early 30s. I used to pull them out until that got to be a losing battle.