In the Colonial Times, work and life went hand in hand. If you were alive you were working. There was no retirement.
During the Industrial Age, leaders in business and government discovered they needed a mechanism to stimulate economic growth and facilitate the replacement of older, less efficient workers with younger, cheaper ones. Mandatory retirement for older workers became the preferred mechanism. So, this is where your question is answered, but I'll fast forward to the era where retirement became a duty and then it became something you looked forward to.
A decade before the Great Depression hit, the foundation for "youth culture" began to develop in the Roaring 20's. At the time, things were moving forward very fast, and it was seen as a progressive time. The older generations were stubborn to this change and as a result the mentality towards older folks were that they were tired, too traditional, close-minded, etc. In the end, the easiest solution to chronic unemployment in the economy seemed to be the retirement of all the older workers who could not keep up physically or mentally. It was their civic duty to get out of the way.
So, after WWII, we entered an age where retirement was cherished. That mentality that old folks needed to step aside and let the young folks take the torch started to seem nice. Before, it was degrading but now it was something you deserved for all of your hard work. I also want to add that the Great Depression forced the government to focus on the younger generation going to work and someone had to take the fall, which were the older folks. It's because of the war that America went into full employment. After the war, the young soldiers returned and they needed jobs. So, old folks retired and women (who were also at work) became homemakers, which is where we would enter that "Nuclear Family" era.
Here are a few sources you can check out to read more in depth: