Unless it manifests itself in a "driving 30 on a highway" kind of way, I doubt it will become an issue.  The only way I do see it applying is in the old-guard/new-guard argument.

Who knows how age will play out in this campaign? It probably won't be a major factor but it will - I think - guarantee that a vigorous, relatively young man will be McCain's v.p. selection.

A more serious take on the problem noted below , from Time 's Scherer : McCain's campaign is

anonevent, McCain risks making this an issue with his VP choice. If he picks a Dan Quayle, his age will weigh heavily against him, in that he is reasonably likely to die in office if elected, and a less-than-excellent VP choice will make McCain look much too risky to some voters on the basis of his age. This is why McCain needs to choose as his VP someone who makes people say, "Hey, you know what, she would make a pretty good president on her own. What an exciting choice!" Anything less than that reaction and the VP puts McCain's age into starkly negative contrast.

"Things Younger Than McCain" Ha. Whippersnappers better shape up if they think disrespecting their elders is gonna get them a comfy place in the Last Will and Testament.

[williamyard stumbles to armchair, retrieves glasses from where they fell]

Now where the hell are my pills?

[mutters]

God damn doctors can't fix a prostate and they can't fix a bum knee and they can't fix insomnia and...wh...what?

[farts violently]

Oh! There she is!

[hobbles to window, peeks out from behind curtain]

[Across the street, a comely college-aged girl pedals bike into driveway and dismounts. She bends over to remove some debris stuck in the bike's chain.]

Ahhhhhhh, yes....[licks lips]...Why, if it weren't for this arthur-itis, I'd show *her* a thing or two. Heh, heh, heh...

[wipes spittle from corner of lip]

ROFL

I think McCain's age will be more important to older voters. The age argument is one of perception for Obama, i.e. he's young so he can't have much experience.  The problem for McCain with older voters in exactly what that Murtha guy said:  the job of president is too much for guys their age.  I bet many older voters will look at McCain and see themselves and how much they feel their own age, even if they are more active than most.  

At the risk of sounding cold-hearted, John McCain suffered as a POW in ways that don't all heal with the passage of time.  A man of 75 (which he would be after 2 1/2 years in office) doesn't have the reserves of stamina to call upon that a man of 65 would.  And that's not taking into account the broken limbs, torture and skin cancer he endured after 7 years without sunblock care of the Vietcong.  BTW, the Boston Celtics are younger than John McCain, too.