The ancient Greek writer Herodotus, often called the first historian, spoke of how memories live for three generations. For that is how long there are those who still knew of the humans who lived that history first-hand.
I remember a lecture on death and dying that spoke of that being the divide when our deceased relatives become ‘ancestors’.
The last First World War veteran passed in 2009 at age 110.
The last Civil War veteran passed in 1956.
The last Revolutionary War veteran passed in 1866.