Actually, they dropped dead much earlier than that.
Only 53.9% of men and 60.6% of women made it to 65. https://www.ssa.gov/history/li...
Well duh, they dropped dead before reaching their 20s.
Straight quote from your link: "Life expectancy at birth in 1930 was indeed only 58 for men and 62 for women, and the retirement age was 65. But life expectancy at birth in the early decades of the 20th century was low due mainly to high infant mortality, and someone who died as a child would never have worked and paid into Social Security. A more appropriate measure is probably life expectancy after attainment of adulthood."
And then they show of all the 21 yo men alive in 1915, 54% of them reached 65 yo (in 1960). Those had 13 more years of life expentancy. So it's retire at 65, drop dead at 77 for that cohort.