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.

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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.”

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.

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