old: advanced in years, and so a senior whose age commands respect or authority
At its core this word simply means old, far along in life. The same word that describes a gray-haired man or woman is the one Israel uses for its leaders, because in this culture age and standing went together: the people who had lived longest were the ones trusted to judge, advise, and speak for the community. So the single word slides naturally from a plain fact about someone's age to a title for an office.
The descriptive use names anyone well along in years, an old man or an old woman, with all that great age suggests in the Bible, both honor and frailty. The institutional use, almost always in the phrase the elders of a place or people, names the recognized body of senior leaders who decided cases at the town gate, represented the nation, and acted on its behalf. These are not two unrelated words but one word seen at two distances: the elder is, by origin, simply an old one.
A narrower extension uses it for the most senior figure inside a single household, the chief or oldest servant who runs the master's affairs. Here too the thread is the same: the eldest is the one in charge.
Which shade is in view is usually clear from context. Standing alone and describing a person, it means aged; in the set phrase the elders of Israel or the elders of the city, it names the office; spoken of a household, it points to its senior member.