What is a form of oligarchical rule in which leaders are much older than other citizens called?

A gerontocracy is a form of oligarchical rule in which an entity is ruled by leaders who are significantly older than most of the adult population. The ancient Greeks were among the first to believe in this idea of gerontocracies; as famously stated by Plato, "it is for the elder man to rule and for the younger to submit".

However, these beliefs are not unique to ancient Greece, as many cultures still subscribe to this way of thinking. Often these political structures are such that political power within the ruling class accumulates with age, making the oldest the holders of the most power. Those holding the most power may not be in formal leadership positions, but often dominate those who are. In a simplified definition, a gerontocracy is a society where leadership is reserved for elders. The best example of this can be seen in the ancient Greek city state of Sparta, which was ruled by a Gerousia. A Gerousia was a council made up of members who were at least 60 years old and served for life.