We know he is old, he has admitted he is old, and he has even joked about it! The comedians have told us how old he is, even the media has slipped it into their stories as sly comments when they don't outright state it.
McCain embraces his joke line! It is a smart strategy similar to using the honey pot theory (understanding the Honey Pot Theory), sucking the comedians, media, and his opponent into a known flaw that most Americans don't consider a game ender.
Obama, either out of fear or out of arrogance, has gone out of his way to deny every single perceived error or flaw he may have. Doing so makes it appear that he really thinks he is a perfect and flawless god.
He lost the ability to pick the laugh line -- how he was defined! Democratic president candidates always make this same error, either out of fear of appearing less then perfect or, as it looks more and more every year, out of arrogance of actually believing their own press. They refuse to allow any area open for attack by the Republicans thus opening all areas for attack. Eventually some of the game changers actually do stick.
The comedians were short on material for jokes (while they had tons of old people jokes for McCain) leaving them struggling for something -- McCain gave it to them, Obama is an arrogant, empty 'celebrity'.
Because Obama is unable to laugh at himself, and because he was unwilling to embrace a benign flaw early, he set himself up for a death by a thousand cuts as everyone struggles for a way of describing him.
He has appeared arrogant and has demonstrated that he has a glass jaw and in a weakened candidate who can't laugh at himself that often leads to unforced errors.
This has made him vulnerable to a knock out punch. A punch at a time when he is vulnerable and lacking the ability to laugh it off because by that time doing so would be so out of his carefully crafted character that the general public won't believe a word of it.