I'm not sure it's quite as wrong as you say though. If the reason the life expectancy of a 65 year old is higher is that they are post-selected for not having died earlier, then it's the case that there were a bunch of people paying social security taxes that were dying before retirement. So there was more money for each one of those 13.9 years, because the people who survived were still able to use the cash put in by those that didn't. If you converted this into some figure that represented how many person-years of retirement the taxes from the average career supported, it would be closer to the three years, not the 13.9 years.