Jokes about John McCain’s age have become staple fare for US late-night chat show hosts. David Letterman quipped that the senator was old enough to remember when Iraq was known as Mesopotamia, while Conan O’Brien claimed his Secret Service codename was “Enlarged Prostate”.

Appearing on Mr O’Brien’s show last week, the Republican presidential candidate seemed happy to play along with the joke, pretending to nod off when the host said filming had been rescheduled to accommodate his guest’s afternoon nap.

For many voters, however, Mr McCain’s age is no laughing matter. Polls show that up to a quarter of Americans believe him, at 71, to be too old to be president.

When the Pew Research Center asked voters last year what characteristics they wanted in a president, more were hostile to the idea of an elderly president than a black president. That suggests age could be a bigger obstacle for Mr McCain than race is for Barack Obama, his African-American opponent.

The Arizona senator will turn 72 the day after Mr Obama officially accepts the Democratic nomination next month at the age of 47, highlighting the stark generational divide between the candidates. If Mr McCain wins, he would become the oldest first-term president in US history, more than two years older than Ronald Reagan when he took office in 1981.

The issue of Mr McCain’s age has also sharpened attention on his choice of running mate, since voters would be particularly anxious to know the vice-president was qualified to take over. Reports this week suggested a decision was close, with Tim Pawlenty, the Minnesota governor, in the frame although Mitt Romney, the former Massachusetts governor and Mr McCain’s vanquished presidential rival, is still considered the frontrunner.

Concern about Mr McCain’s age is heightened by his history of skin cancer. According to health records released to the media in May, he has had four potentially life-threatening mela­nomas removed, as well as an unspecified number of less serious skin cancers.

He also carries scars from his wartime ordeal in Vietnam, where he broke a leg and two arms when his naval aircraft was shot down. He later suffered torture during five and half years as a prisoner of war.

No other candidate has released comprehensive health records in the way Mr McCain has. But the way the 1,200 pages were presented to the media – for three hours, on the eve of the US memorial day weekend, to a pool of 20 reporters who were not allowed to copy the documents or take them out of the room – attracted criticism. Even so, the rec­ords dispelled rumours on his health, showing he had not been treated for mela­noma since 2002 and finding he was in “excellent health”, with “extraordinary energy”.

The focus has more recently shifted to scrutiny of his mental sharpness, as critics pounced on a series of verbal gaffes to portray him as bumbling and confused. In the past three weeks he mixed up Iraq with Afghanistan, Somalia with Sudan, Russia with Germany, and Czechoslovakia – a country that no longer exists – with the Czech Republic.

McCain officials point out that Mr Obama has made his own slips, on one occasion overstating the number of US states by seven, and that recorded mistakes are inevitable over the course of a long campaign.

Wary of being labelled ageist, the Democrats have avoided direct attacks on the issue but Republicans accuse them of using code to highlight Mr McCain’s age. In one speech, for example, Mr Obama praised his opponent’s “half-century of service to this nation”.

Gerald Davison, an expert on ageing at the University of Southern California, says age alone should not rule out Mr McCain. “Someone who is 70 could perform as well at a given job as a 40-year-old, or as poorly as a 100-year-old,” he says.

Republicans hope concern over his age will be offset by respect for his experience. “I’m older than dirt and I have more scars than Frankenstein,” Mr McCain likes to joke. “But I learned a few things along the way.”

The election of a septuagenarian to the Oval Office would certainly be in keeping with the growing role of older people in US society. People 85 and above are the fastest-growing segment of the US population.

Opinion polls show a deep generational split among voters, with two-thirds of under-30s planning to vote Obama. Mr McCain has a narrow advantage among the over-55s.

The Democrats hope a large turnout of young voters in November will catapult Mr Obama to the White House. But the young are less reliable as voters than Mr McCain’s older base: the proportion of under-30s saying they would definitely vote in November has dropped to 46 per cent, from 66 per cent in March.

Leonard Steinhorn, a professor of communications at American University in Washington, says Mr McCain’s problem is less his age than his detachment from contemporary culture, highlighted by the recent admission that he rarely uses a computer. “It’s not his age, it’s whether he understands the society he aspires to lead,” said Prof Steinhorn.

Copyright The Financial Times Limited 2018. All rights reserved.

Comments have not been enabled for this article.