grey or white hair, and by extension the advanced old age it marks
The word points first to the literal grey or white hair of an aging head, then slides naturally to the whole season of life that grey hair announces. Hebrew keeps one word for both, so "grey hair" and "old age" are not two separate ideas here but the two ends of a single one: the visible sign and the stage of life it signals.
In Israel grey hair was an honor, not something to hide or dye away. It is called "a crown of glory" and the splendor of the old set against the strength of the young. Reaching it is a blessing: the great forefathers die "in a good old age," and long life crowned with grey hair is treated as a mark of God's favor. The plainer word for old age (זִקְנָה) names the same stage without this picture of the whitened head.
The same word carries a heavier note in a fixed saying about death. To bring someone's "grey hairs down to the grave" is to send an aged man, or the old self he has become, down to death in grief or in violence. Here the grey hair stands in for the whole person in his final years.
Once the word leaves people entirely. In Job the churned white wake trailing the sea monster makes the deep itself "look hoary," as though the ocean had grown grey-headed.