Gerontocracy

Etymology

From Greek gérōn ("old man," stem geront-) plus -kratía ("rule"): "rule by the aged." The root also survives in words like "geriatric." It was long an anthropologist’s term for elder-led tribal councils before becoming a critique of political systems dominated by their oldest members.

noun · /jair-un-TOK-ruh-see/
A society or institution governed by the old, in which the eldest hold most of the power.
What it names
It gives a name to the way seniority can quietly harden into rule, so that the interests of the young go structurally unrepresented.
Example
With the average senator well past seventy, commentators warned the chamber was sliding toward gerontocracy.
Related idea
Patriarchy organizes power around gender and the father’s authority; gerontocracy organizes it around age. The two often overlap in the figure of the elder statesman, but they answer to different logics.