Interestingly, the one candidate (or surrogate, pre-Dean) who most often raises the age issue is none other than its supposed victim: John McCain himself.
"Older than dirt, more scars than Frankenstein," is a favorite line of his. (though he did botch it at least once.)
So, obviously, McCain is well aware that his age, and the concomitant off-spring issues spawned by it--his death in office, his forgetfulness or seeming befuddlement, the unusual importance of his VP running mate and so on--are issues that will always already be there in voters minds.
His age, like Hillary's gender and Barack's color, are facts, period, facts visible to the naked eyes of voters. They can't be spun or otherwise hidden from public view. And to some, they would be considered "negatives."
Maybe McCain's strategy of raising the age issue himself at almost every opportunity is his way of disarming or charming his adversaries by reducing the issue to the category of joke.
After all, jokes aren't made about Iraq, nor about the financial/housing crises---they're serious issues. But "age?"
It's the stuff Letterman, Leno & Co. can mine night after night, without concern for any negative sympathy-based responses from their audiences. The late-night comics would never ridicule someone whose has lost an arm or leg, or suffers from any physical malady.
The dems can beat McCain on the real, non-laughable issues; they run the risk of a sympathy backlash if they hit him too hard on his age. He can joke about it himself; Letterman can do the "10 Ways You Know John McCain is Awake" monologue every night---but gratuitous, duplicitous remarks such as Dean's only feed the public's perception that politicians are slime.