I've always wondered about this, as well.
My parents showed gray in their mid to late years.
Mom turned from auborn red to gray overnight (so it seemed), in her late 40's and dad, who's now 80, still has alot of his natural dark color remaining, although been pretty bald since his early 20's.

My siblings fall close to suit. There's 10 of us, with 5 brunettes and 5 reds, (I'm one of the reds). Seems that the brunettes have gone gray way early in their years, while the reds have lost hair (brother's, anyway...lol).
My younger sister of 3 years is also red, yet she is almost completely gray now. The other younger sister of 5 years is brunette, and has shown a gradual gray beginning years ago. She colors the gray out.

Out of all of us siblings, so far I'm the only one who hasn't shown any sign of gray whatsoever, (which I find odd, as out of all of us kids, I've had a rougher life by far than any of the others). So, I'm assuming that I'll be either like my mom was...go gray overnight, or like dad is....slight gray in late years. Either way, it's a "knock on wood" for mez...lmao.
I'm just in delay mode, I guess...lol, (though that's not surprising..I was always behind on everything I've done since the age of 3).

Shangrala