I hadn't expected to get sucked into presidential campaign politics during my first few posts. Nevertheless, here I am. Please excuse this brief indulgence.
The people who criticize McCain for being too old to run for President are only half right. Yes, he is at risk of not living through his first term, were he elected. (Or his second, for that matter.)
However, the other problem with his age is not that his mental faculties may be slipping. It's a concern if it were true, but there's a bigger potential problem.
This is the last chance for McCain to run for President. In fact, it's the last chance for him to be a major figure in American politics, if he follows his current personal trajectory to its likely destination. If he were not aggressively vying for the Oval Office, the Democrats would likely control both the executive and legislative branches. The Republicans would continue to sink--along with John McCain, for his support of the failed Bush presidency, not to mention recently throwing in his lot with the Kulturkampfers in his party.
McCain is surely aware of his own mortality, so he must know that this is his last shot at the White House. Look at his behavior in that light, and a lot of what he does might make more sense. That's not the same as saying he's justified in what he's doing lately--far from it. Other leaders have stared Death in the face, too.