On this week's edition of US TV's The Daily Show, presenter Jon Stewart asked Hillary Clinton a leading question.

Did she have a favourite shape for an office? For example, did she like it to have corners? Clinton appeared to think carefully. "I think that the world is so complicated, the fewer corners that you can have, the better," she answered, deadpan, and then broke into peals of loud, merry laughter.

So there you are: Hillary's strongest hint yet that she intends to run for the famous Oval Office in two years' time.

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Or perhaps she really was just joking. Either way, speculation about a 2016 presidential run by the former First Lady is as hot as ever. Last time round, when she was vying with Barack Obama for the nomination, all the talk was of her gender. But if Clinton decides to run in 2016, she will be flying the flag not only for women but also for what Americans call "seniors"; she will be showing what people of pensionable age are capable of. And a good thing, too. In a world where ageism is rife, and life expectancy is growing rapidly, she will be doing everyone a favour.

Every peer group brings social pressures and among the world leaders who are the US president's peers, the trend is to be under 60. The figures on age make for interesting reading. In the western world, leaders are more youthful than they once were; David Cameron is just 47.

The average age of leaders of the G7, the world's top industrialised countries, is 53, with Germany's Angela Merkel the oldest at 60 and the outrageously young Italian Prime Minister, Matteo Renzi, the whippersnapper of the group at 39.

If you look at leaders of the larger G20 group of the world's leading economies (bear with me), the average age is higher, at 60. But look at their ages on assuming office and the average falls to 55.

Hillary Clinton, if she wins the presidency in 2016, will be 69.

Is that a problem? Some (principally her political enemies) would nod vigorously but, surely, the key issues are health and stamina, not age.

There are some 69-year-olds who would not be able to sustain the pace of presidential life but Clinton knows better than anyone what she would be letting herself in for. This is a woman who has been working 15-hour-days for 20 years.

Ageism has played a part in recent presidential contests, as it has in British politics (where Menzies Campbell's leadership of the Liberal Democrats was cut short because of it). Republican candidate John McCain was 71 when he ran for the White House and the date on his birth certificate was a campaign issue, though he has proved himself sharp and energetic in the six years since.

Speculation about Clinton's health has dogged her ever since she had a severe intestinal infection and fainted because of it back in 2012, sustaining concussion and ending up with a blood clot, but she confirmed last month there had been no lingering effects and that she would release her medical records to prove it if she decided to run in 2016. Naturally, her political opponents have sought to insinuate otherwise, but they seem to have no grounds for doing so.

Clinton's gender combined with her age is an added challenge. She would be breaking the mould as an international stateswoman in her 70s.

But why should we fear age? Ronald Regan took office at nearly 70 and left at 77 and is regarded by Republicans as one of the greats. Our notions of what constitutes old age are changing rapidly in any case.

Retirement at 70? Even that might seem early in a few decades' time. Strong performance in office is what will change public perceptions about age; and who better to do that than the redoubtable Mrs Clinton?