Mr McCain frequently makes jokes about his age, and his scars
A quarter of a century separates John McCain and Barack Obama.
When Mr McCain was taken prisoner in the Vietnam War, his Democratic opponent was just six years old.
Now, Mr Obama is a basketball-playing, Blackberry-obsessed 46-year-old - at ease with the latest pop culture references.
John McCain, by contrast, is physically constrained by his wartime wounds, wary of the internet, and more likely to quote the Beach Boys than Jay-Z.
He turned 72 in August - and if he is elected, he would be the oldest man ever to become US president.
And that worries voters.
"I think there are two sets of concerns," said Andrew Kohut, president of the Pew Research Centre.
"One, will an older candidate, or older president, have the energy and sharpness to do the job well? And secondly, there's a concern about whether an older president will have old ideas."
His age has already become the the butt of jokes - one popular internet site lists the things that are younger than John McCain.
But if you can't beat 'em, join 'em - the Arizona senator has decided to play along.
"I'm older than dirt and I've got more scars than Frankenstein," he joked on David Letterman's late-night talk show.
To paint McCain with a brush that says 'I can look at his age and know everything I need to know about him' is just as wrong as looking at Obama and saying 'I can look at his skin colour and know everything I need to know about him'
Dr William Thomas Geriatrician, University of Maryland
It is a tactic mature Republican candidates have tried in the past - with varied results.
Ronald Reagan - 73 years old at the time - managed to pull it off in the 1984 campaign.
"I will not make age an issue in this campaign," he famously told rival Walter Mondale during a televised debate.
"I'm not going to exploit for political purposes my opponent's youth and inexperience."
But in the 1996 election, 73-year-old Bob Dole was less successful.
A clip of him tripping and falling off a stage got repeated airplay, and did him no favours with voters.
But whatever effect John McCain's age has on public perceptions of him, it is not helping him close the enthusiasm gap.
Young voters seem to have far greater excitement for the Obama campaign.
Not first choice
The attendance is high at a Young Republicans meeting in Washington DC.
How US voters see age as an issue
But - they admit - their energy levels are not.
On a scale of one to 10, how excited are they about John McCain?
"I have to say five," says Mark Hill.
"Because I get mixed feelings from people, I have always had to defend him... But it's probably five."
Mr Obama was six years old when Mr McCain was taken prisoner in Vietnam
For many here, Mr McCain was not their first-choice candidate.
His age and style have made him harder to identify with.
"I think that his message is a little bit more traditional and more pragmatic which makes it harder to appeal to voters on an emotional level," says Katherine Seacrest, a graduate student.
"And I think that you have to have that if you want to inspire people and get them motivated... Making sure Barack Obama doesn't get the nomination... motivates us more than John McCain's message. Which is a problem in and of itself."
But - Young Republican or partisan Democrat - is it fair to make a judgement on John McCain based on his age?
Dr William Thomas, a geriatrician and professor at the University of Maryland's Erickson School of Aging Studies, says it is not.
"One of the most common prejudices we have in American society is a prejudice against age," he said.
"Young is always better... old age is bad... that's the prejudiced attitude. And to paint McCain with a brush that says 'I can look at his age and know everything I need to know about him' is just as wrong as looking at Obama and saying 'I can look at his skin colour and know everything I need to know about him' - it's not fair."
At the daily poker game at Leisure World Retirement Centre in Silver Spring, Maryland, some at least agree that age should not be a barrier to becoming president.
"It depends who he picks for vice-president - I don't think his age makes any difference," says 82-year-old Al Bramow, a Leisure World resident.
But Mr Bramow's fellow resident, Bill Hudson, disagrees: "If he has to pick a special vice-president then you're worried about his age. Sure you are."
John McCain may prefer playing craps than poker, but, games aside, he needs to convince voters - of all generations - that electing a 72-year-old to the White House is not too much of a gamble.
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