In an earlier, harsher America it was not uncommon for people to work well into old age. In 1945, about half of Americans 65 and over were still in the labor force. And that was at a time when most work was physical in nature.
Then came the great rise in postwar prosperity, with the spread of defined benefit pensions (often won by unions), the expansion of Social Security benefits, the creation of Medicare, and big leaps in housing wealth that provided nest eggs for retirement.