I’m sure it isn’t helping Obama that his campaign strategy has been to attack McCain on age. The democrats have been downright rude about how the “elderly” are not capable.
That's an interesting issue, and Obama certainly has been attacking on it, as in at that age you're automatically senile. I doubt that helps, most older folk don't look at age 70 as the onset of senility.
On the other hand, if they'd emphacized the stresses of the job, who knows, it might resonate. I've no doubt of McCain's ability to do the job, but I'd speculate that many seventy somethings might consider it beyond the capabilities of themselve and friends. But outright attacks on his, and their, mental facilities, I doubt that works.