Up to 80 years ago you retired when you could not physically work. That could have been as early as 50s for physically demanding farm and factory jobs. Then Socially Security and pensions allow people to retire at the arbitrary calendar age of 62. This results in a senior leisure class of a quarter of the US population. Many of those could work at sedentary service and desk jobs. And some continue to work out of psychological and financial needs.

(I say 62 even though full retirement age is 67 because half retire when they can first draw partial Social Security.)