These days most Pres serve two terms. 8 years. The stat you quoted means one half of the people in McCain's position live less than 12 years. That leaves a very decent chance that if McCain is elected, his Vice President will be elevated to the Presidency.
Not a sure thing, but a very real chance. Especially since McCain has had recurrent melinomas, and because a person does not have to die to become functionally unable to fulfill one's duties as President.
This means that it actually matters who McCain chooses as VP. And it is a pity he did not choose someone who doesn't claim as her only foreign policy experience that she can see Russia from parts of her state. Among other failures of experience and knowledge.
NOTE: McCain has NOT said he will serve one term.
"He was more categorically opposed to the idea of running for one term when he was asked about it at a Fox News candidates’ forum in January.
”No, because I think then you’re the lame duck, you’re quacking on Inauguration Day,” he said. ”But look, I would point out that when Ronald Reagan won the Cold War, he was in his second term older than me. I think that’s pretty good, isn’t it? Look, I’ve got the vigor. I’ve been - everybody has seen me here on the campaign trail. We work 16, 18 hours a day.”