mccain would be 72 years old by inauguration day. using the social security actuarial tables this is what his odds look like:
odds of surviving (odds of dying) each year of the presidency:
first year: 96.5% (3.5%)in other words, there is approximately a one in three chance that mccain would not serve to the end of his second term.
second year: 96.2% (3.8%)
third year: 95.9% (4.1%)
fourth year: 95.5% (4.5%)
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total odds of surviving (dying) by the end of his first term: 85.0% (15.0%)
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fifth year: 95.0% (5.0%)
sixth year: 94.5% (5.5%)
seventh year: 94.0% (6.0%)
eighth year: 93.5% (6.5%)
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total odds of surviving (dying) by the end of his second term: 67.1% (32.9%)
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of course, those are just statistics taken from the average man of his age, without taking into account any of the particulars of john mccain (e.g. whatever toll his time as a POW may have taken). nor does it take into account the health benefits and costs of being a sitting president. thus it doesn't take into account the better-than-average health care a president would get, nor does it take into account the added stress of the job. it also doesn't take into account the bubble of protection that surrounds a president (presidents, for example, are probably a lot less likely to die in a car crash than the average person), or the added dangers of being a president (the average person is not generally a target of assassination).
but all else being equal, there's roughly a one-in-three chance that an average guy of mccain's age wouldn't make it to 2017, when mccain's second term would be scheduled to end.
unless, of course, i screwed up the math somewhere. we can never rule that out.