County militias certainly existed prior to the Revolutionary War, and were continued during it.
Except in certain frontier circumstances, such militia organizations would not have seen active duty after 1763.They did have chains of command and the complement of officers one would expect.
One major exception was Dunmore's War in 1774, where Lord Dunmore's troops included men from all over East Virginia plus what was then called Augusta County militia (from a very broad area, mainly northern present-day WV and SW Pennsylvania).Among this group were some who were 60-ish, anyway.
One of the famous old-men items during the Rev. War was an experienced and trusted militia officer who took over his son's Company at the reputed age of 76 years (after the son deserted to the enemy).This certainly was not to happen often, and the really able-bodied in their 70s would be quite uncommon.But not impossible.