The meaning of "greybeard" have moved from being literally, to current be figuratively. It doesn't mean that the person is A) old, B) man C) have a beard and D) that the beard is grey.

It simply means someone experienced/being at something for a long time. The greybeards at a company can be all women of the age 26, but if they been with the company for 5 years when the company got started 5 years ago, they are the greybeards of that company.

Might as well say "veteran" or "old-timer", although "old-timer" would imply actual old age, "veteran" implies nothing like that.