Average life span is a bad number to use because it improperly counts the affect infant and childhood mortality have. A child dying has no affect on the financial status of retirement plans. It's sad for the parents, but since the kid never put anything into SS he doesn't affect the accounting.
A better number would be life expectancy of someone who has hit 18 and is entering the work force. I expect that number is significantly higher than the 63-65 ages we often see for life expectancy when SS was started.
Makes no difference which set of figures you use. Child mortality stopped being an issue stateside during the 20th century. Average life span went from the low 60's to the low 80's. That's 20 more years for SS to carry, which is a really heavy burden, considering the pay-as-you-go nature of the program.
Of course, if SS were structured so that people got what they put in, instead of getting more than they paid in, SS wouldn't have a solvency problem. At the same time, without the getting-something-for-nothing aspect of SS, it wouldn't be anywhere near as popular as it is today.
Most who are considering early retirement could have retired at 29 if they hadn’t been so foolish in their youth.