Best Resource for Natural Hair Color Rejuvenation

I haven’t noticed this so much in women, though it may happen that way too – I think it’s just more noticeable with guys.

When men get older, I notice some of their hair goes completely gray or white, but their eyebrows remain dark, or the same color as their hair used to be. Why don’t they always go completely gray too?

Any ideas? Does it have to do with genes? Because I know some people go entirely gray.
If it is genes, does that mean the genes that determine hair and eyebrow color are separate ones?

genes. dna. duh.