THIS. It's probably an even more significant issue in terms of mortality stats. We're not just dealing with "Average Joe" here -- these guys were generally chosen because they were in top physical and mental condition... physically probably in the top 5% of the population, if not higher. It shouldn't be surprising at all that most of them live to their mid-80s or more.
Sibling post by someone else points out that they used the low-Earth and grounded astronauts-in-training as a control group, so I now assume that accounted for that. BUT: your/our point still stands that you'd expect astronauts of any mission to generally be in better shape than the general population, and live longer in general.