"In a moment," says the White HouseThe official residence of the US president, in Washington DC. Burnt by British troops in 1814, it was rebuilt and painted white to hide the smoke stains. guide, "we will come to the Oval Office, where the president's most important work is done. But first I want to show you something not many people get to see." He opens a small door with a flourish. "Here it is, ladies and gentlemen - the presidential walking-stick collection!"

This may be fantasy, but there is one inescapable fact: both the favourites for next year's presidential election are old enough to be your great-great-grandfather. Joe Biden was born during World War Two, and Donald Trump just after it. If Biden wins and completes a second term, he will be 86.

A recent opinion poll¹ found that 73% of Americans said they were seriously worried that Biden was too old to be a competent president, while 68% questioned whether he could understand the next generation's concerns. Many of his own party would prefer him not to run for office again.

But at a fundraising event on Monday, Biden claimed that the main thing about his age was that it gave him wisdom and experience. "When this nation was flat on its back," he said, "I knew what to do... When democracy was at stake, I knew what to do."

While Biden's critics emphasise embarrassing photos of him stumbling on steps or falling off his bicycle, supporters point out that on his recent trip to Asia he travelled over 18,000 miles in five days and met more than 20 foreign leaders.

Nikki Haley, who is competing with Trump for the Republican nomination, has called for politicians over 75 to be tested for mental competence. But according to one expert, Professor Hans Forstl: "The demands on a head of state are exceptional in every respect; no test would be able to gauge that mix of fitness, intelligence, experience and wisdom."

The fact is that ageing affects some people far more than others. NeuroscientistsA scientist who studies the brain and the nervous system.  have identified a group called "super agers" who remain as mentally sharp as people far younger than them. It has been suggested that Biden and Trump belong to this.

Some skills actually improve with age. For example, people go on adding to their vocabulary well into their sixties.

Most countries have a leader who is considerably older than the average citizen - and only four have one who is younger: Andorra, Montenegro, Italy and Finland.² Cameroon's president, Paul Biya, is 90.

When Biden was elected at 77, he beat the record for the oldest person to become president - previously held by Trump, who was 70. The youngest president was Theodore Roosevelt at 42.

Winston ChurchillThe British prime minister during World War Two, and later from 1951 to 1955. was 78 by the time he retired as Britain's prime minister. Mahatma GandhiAn Indian political activist and lawyer (1869-1948), who emphasised passive resistance. was still politically active - and even went on hunger strike to stop religious violence - at the same age.

The question of age does not just apply to presidents. Members of the US Supreme CourtThe highest court within the hierarchy of courts in most legal jurisdictions. are appointed for life. One of them, Ruth Badur Ginsberg, was still serving when she died at 87. A 95-year-old federalIn the USA laws are made both by individual states and the federal government, which governs all states together. judge, Pauline Newman, is suing colleagues who want her to retire, saying they are violating the US ConstitutionA document that sets out the fundamental rules according to which a country is governed, which are usually difficult to change..

Yes: Being the leader of the US is one of the most exhausting and responsible roles in the world. Only someone with a high level of mental and physical fitness should be entrusted with it.

No: Modern medicine means that people can continue to function efficiently at a much greater age than in the past. The experience and perspective that come with age should not be underrated.

Or... Old people are frequently written off when they are still highly competent and able to contribute to society. The fact that the US elections has focused attention on this issue is to be welcomed.