A gerontocracy is a form of oligarchical rule in which an entity is ruled by a small clique of leaders, in which the oldest hold the most power. Those holding the most power may not be in formal leadership positions, but dominate those who do: in the time of the Eight Immortals of Communist China, it was quipped, "the 80-year-olds are calling meetings of 70-year-olds to decide which 60-year-olds should retire".

Such a form of leadership is common in communist states in which the length of one's service to the party is held to be the main qualification for leadership. It is also common in religious theocratic states such as Iran in which leadership is concentrated in the hands of religious elders. (Despite the age of the senior religious leaders, however, parliamentary candidates in Iran must be under 75.)

Outside the political sphere, gerontocracy may be observed in other institutional hierarchies of various kinds. Generally the mark of a gerontocracy is the presence of a substantial number of septuagenarian or octogenarian leaders—those younger than this are too young for the label to be appropriate, while those older than this have generally been too few to dominate the leadership in numbers. The rare centenarian who has retained a position of power is generally by far the oldest in the hierarchy.

Gerontocracy generally occurs as a phase in the development of an entity, rather than being part of it throughout its existence. Opposition to gerontocracy may cause weakening or elimination of this characteristic by instituting things like term limits or mandatory retirement ages. Judges of the United States courts, for example, serve for life, but a system of incentives to retire at full pay after a given age and disqualification from leadership for those who fail to do so has been instituted. The International Olympic Committee instituted a mandatory retirement age in 1965, and Pope Paul VI removed the right of Roman Catholic Cardinals to vote for a new Pope once they reached the age of 80.

On the other hand, gerontocracy may emerge in an institution not initially known for it. The Church of Jesus Christ of Latter-day Saints was founded by a 24-year-old man, who in 1835 constituted the first Quorum of the Twelve Apostles with members ranging in age from 23 to 35. Once it was established that succession to the church presidency derived from longest tenure in an office held for life, the hierarchy aged markedly, and with the growth of the church the age at which officials were named to the highest bodies continued to rise. Six church presidents have held office past the age of 90, and as of 2005 the church is actively led by a man who remembers the day his father replaced the family horse-wagon with a Ford Model T.

Gerontocracy's strength is seen as its stability, which can be more appropriate for institutions that teach principles that do not vary over time. In institutions that have to cope with rapid change, the decreased faculties of the aged can be a handicap in providing effective leadership.

Gerontocracies of various kinds can be expected to rise and fall. The increase in human longevity may change the definitions thereof, as ages formerly typified by senescence and survived to by a small proportion of the population are reached by larger numbers of healthier people. For this reason mandatory retirement ages may be raised or face increasing opposition. When pope John Paul II was elected, about one tenth of the College of Cardinals was over 80, but today the proportion has surpassed one-third.