What is rule by the oldest called? Something -ocracy.?

Answer 1

Gerontocracy.

Answer 2

You're thinking of Gerontocracy.

"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. Often the political structure is such that political power within the ruling class accumulates with age, so that the oldest hold the most power. Those holding the most power may not be in formal leadership positions, but often dominate those who are.

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 potentially be a handicap in providing effective leadership..."

Answer 3

gerontocracy

Answer 4

geriocracy