Is ageism finally dead in UK politics?

If Sir Vince Cable becomes Lib Dem leader - and he faces no challengers at the moment - we will have the oldest line-up of party leaders in the Commons since the 1950s. Has British politics finally got over its obsession with youth?

Politics is one of the few fields where being in your 40s is seen as being young.

When Tony Blair became prime minister in 1997 at the age of 43, much was made of the fact that he was the youngest occupant of Downing Street since 1812.

The first opposition leader he faced, William Hague, was even younger, at 36. And he wore a baseball cap to prove it, which, on reflection, was probably a mistake.

When Charles Kennedy took over as Lib Dem leader in 1999, at the age of 39, it was the youngest line-up of leaders the Commons had ever seen.

Two decades later and leadership has skipped a generation upwards. Sir Vince is 74, Theresa May is 60 and Jeremy Corbyn 68.

As Channel 4's Michael Crick has pointed out, this is the oldest collection of party leaders the Commons has seen since April 1955, when 80-year-old Sir Winston Churchill was prime minister, up against Labour leader Clement Attlee, 72, and 71-year-old Liberal leader Clement Davis.

Churchill and Attlee were political giants, but they were in the twilight of their careers and, in Churchill's case, in poor health. Both were under constant pressure to stand aside for younger men.

But the idea that political leaders should always be vigorous go-getters, open to new ideas and the latest trends, ideally with a camera-ready wife and children, did not really take off until the decade that followed, when being young was suddenly all that seemed to matter.

Youngest UK party leader and prime minister: William Pitt, the Younger, a Whig who was regarded as a Tory by opponents, became prime minister in 1783 at the age of 24

Oldest UK party leader and prime minister: Liberal Party leader William Gladstone, pictured, resigned in 1894 at the age of 84. Sir Winston Churchill was 80 when he retired as prime minister and Conservative leader in 1955

Oldest US president: At 70, Donald Trump is the oldest person to enter office. Ronald Reagan, who was 77 when his presidency ended in 1989, was the oldest person to hold the office

Youngest US president: Theodore Roosevelt, who became president at the age of 42 after the assassination of William McKinley in 1901, was the youngest person to assume the office. John F Kennedy remains the youngest person, at 43, to be elected to the White House

The catalyst was Harold Wilson, who was 47 when he became Labour leader in 1963.

The pipe smoking, avuncular character in a grey raincoat was sold to the public as a youthful, dynamic breath of fresh air.

Not as glamorous, perhaps, as the then US president John F Kennedy, who was just a year younger than Wilson, but certainly a lot younger, and more in touch with the modern world, than 70-year-old Tory leader Harold Macmillan.

It is a short step from Wilson chumming around with The Beatles at the Variety Club awards in 1964 to Tony Blair hosting Noel Gallagher at Number 10.

Being young and hip, or being able to stand next to someone who is young and hip without making a complete fool of yourself, had become a key attribute for aspiring political leaders.

Like his friend and political soul-mate Bill Clinton, Tony Blair was self-consciously youthful in a way that will be instantly familiar to embarrassed teenagers everywhere. He was even pictured walking into Downing Street with an electric guitar.

And although prejudices against female and ethnic minority politicians began to crumble during the Blair-Clinton years, ageism lingered on. Political leaders appeared to come with a self-imposed sell-by-date.

"At the time of the next election, I'll be 67 or 68 and I believe that is simply too old to lead a party into government," said Michael Howard in 2005, as he claimed the then 64-year-old Ken Clarke was too old to succeed him as Conservative leader.

Mr Howard's 39-year-old protege, David Cameron, fitted the youthful bill, and earned bragging rights in the Commons when he told the no-longer-quite-so-youthful Tony Blair: "You were the future once."

He even broke Tony Blair's youngest prime minister record by a few weeks.

But can you have too much of a good (or bad, depending on your point of view) thing?

Smooth, identikit politicians such as Mr Cameron, his coalition partner, Nick Clegg, and their Labour opponent at the 2015 election, Ed Miliband, were all in their 40s with young children.

The stage was set for a grizzled veteran, even if he did not realise it at the time.

"At my age I'm not likely to be a long-term contender, am I?" Jeremy Corbyn told the Guardian, after surprising himself, and everyone else, by getting on to the Labour leadership ballot in 2015.

The wild enthusiasm among young left-wing activists for Mr Corbyn gave the lie to the idea - long-held by broadcasters as well as politicians - that the under-30s could relate only to people their own age.

Across the Atlantic, Bernie Sanders, the 75-year-old senator who lost out to 69-year-old Hillary Clinton in the battle for the Democratic nomination, is another example of a pensioner who commands crowds of adoring millennials.

In the social-media age, authenticity can be prized above all else - and, whatever you think about their politics, Mr Corbyn and Mr Sanders are unspun figures who are comfortable in their own skins. Age and experience are, arguably, part of their appeal.

Age was not a barrier to success for Donald Trump either, who, at 70, is America's oldest elected president, beating previous record-holder Ronald Reagan by a year.

Which brings us back to Sir Vince Cable, who prefers to compare himself to Churchill and William Gladstone, who became prime minister at 82, than the current occupant of the White House, but who has also faced questions over his advancing years.

There were mutterings about the party going from "dad to grandad" when he announced his intention to stand.

The former business secretary, who won back his Twickenham seat at the general election, was having none of it.

"Some of the brightest and most interesting people in British politics recently have been relatively old. You remember Bernie Sanders in America as well? I don't feel old, I feel young and energetic, and I'm very much up for a contest."

Sir Vince has always regretted not standing for the party leadership in 2007, when Sir Menzies Campbell was effectively hounded out of the job for being too old, at what these days might be described as the youthful age of 66.

Sir Vince, who was then 64, apparently feared that he would also be judged too old.

But now, 10 years older, and with the forty-something generation of leaders having departed the stage, that no longer seems to be an issue.