American Social Security had an age of 65, but at the time of its passing life expectancy for American men was 58.
That was life expectancy at birth. It makes more sense when considering retirement system design to consider the life expectancy of people who are at least old enough to be in the workforce.
Of the people who were 21 when Social Security passed around 65% of them survived to at least 65.
Of the people who were born around the time Social Security passed, over 70% of those that made it to 21 made it to 65.
As you can already read sidethread, a life expectancy of 70 means huge majorities of people living well past 70. Every baby who dies at the age of 8 months means half a dozen people die in their 80s. (Or, you know, several dozen people die in their 70s.)
Even if lifespans weren't skewed low, which they are, 50% of all people would live beyond the life expectancy at birth.
I think still today that is a deciding factor, as some economies have high unemployment and there retirement can be cheaper and better than caring for unemployed people at all ages. And other economies have high employment and would even benefit economically from a larger workforce.