His critics worry that he may be too old to become president of the United States, but experts on ageing have concluded that John McCain should not only be fit enough to complete two four-year terms, he should also have at least five years left over for a comfortable retirement.

The Republican presidential candidate will live to be 85, another 13.1 years beyond his 72nd birthday this August and well after his likely political sell-by date, according to the latest US government actuarial tables.

Despite his gruelling personal history as a Vietnamese prisoner of war and later as a sufferer from skin cancer, McCain has “a very good chance for a very long life”, said Professor Jay Olshansky, the author of numerous research papers on ageing and mortality.

Olshansky cautioned that it was impossible to make reliable predictions about individual lifespans. “You just can’t know for certain,” he said. “There are lots of people with long-lived parents and apparently healthy lifestyles who drop dead when they are 40.”

A race like a crackpot Dynasty

The contest between Barack Obama and Hillary Clinton matches any fiction

Multimedia

• Full coverage: US elections

BACKGROUND

• The ascent of Barack Obama, Mr Charisma

• Decay, jobs and guns fall under the spotlight

• Free trade: Clinton/Obama's mistakes

Related Links

• John McCain visits Middle East and Europe

• McCain ‘has 6pc risk’ of dying from cancer

• McCain wins nomination and switches focus

Yet all the evidence from more than a century of US population statistics indicates that McCain belongs to a group that has been dubbed “the alpha geezers”. More than a third of the US population now live to be more than 85 and many of them remain active well beyond retirement age.

A new book, The Longevity Revolution, recently argued that in the coming century the human lifespan will extend to 120 years. Robert Butler, the book’s author, is a professor of geriatrics at a medical school in New York and is still working at the age of 80.

Concern about McCain’s capacity to survive as president has mostly focused on the physical hardships he endured as a captured US navy pilot after his plane was shot down over Vietnam in 1967.

He spent 5½ years as a PoW and was severely tortured. He still has difficulty climbing stairs and cannot raise his arms to comb his hair.

Yet Olshansky claimed that the prison experience would not necessarily affect his lifespan. “Sometimes a bit of stress early in life can be beneficial,” he said.

McCain may also have benefited by not being able to drink alcohol or consume fatty foods at a key period of his early thirties. Any other health problems related to his captivity would probably have manifested themselves by now.

The single most important factor in longevity calculations tends to be the lifespan of the mother, Olshansky added. McCain’s mother, Roberta, is still alive and kicking at 96 and has repeatedly startled the senator’s campaign aides with acerbic comments about the presidential race.

When Chuck Norris, the Hollywood actor, questioned McCain’s stamina, the Arizona senator joked: “I may have to send my mother to wash Chuck’s mouth out with soap.” When a high school student raised the issue at a campaign meeting in New Hampshire, McCain prompted laughter by replying: “Thanks for the question, you little jerk. You’re drafted.”

Presidential history may also be on McCain’s side. At 72 he would be the oldest president to take office for the first time, but no White House occupant has died of natural causes since Franklin D Roosevelt collapsed with a cerebral haemorrhage in 1945. Roosevelt was paralysed from the waist down but hid his disability for more than 20 years.

Ronald Reagan’s age was questioned when he first became president aged 69 and ran for reelection at 73.

In the 1984 campaign he buried the issue when he joked about Walter Mondale, his younger Democratic rival: “I am not going to exploit for political purposes my opponent’s youth and inexperience.”

Some of Reagan’s staff have since claimed that they had detected signs of Alzheimer’s before he left office in 1988, but he remained broadly popular with the American public.

Barack Obama, the 46-year-old Democratic contender, has made subtle references to McCain’s age by praising his “half-century” of service. Yet Olshansky believes that the Democrats would be making a mistake to question McCain’s likely longevity.

“I don’t think it matters because, personally, I think the Democrats are going to win anyway,” he said. “But there’s nothing to suggest John McCain won’t live long enough to do the job. I would not count his age against him at all.”

John McCain has launched a new capaign web ad. View it here

Times Recommends

• Both sides blame US for worsening relations
• Stieg Larsson: losing the plot over his cash
• Odd creature paralysed by incompatible parts