It is the year of the octogenarian. American TV viewers can find Patrick Stewart, 82, boldly going in a new series of Star Trek: Picard and 80-year-old Harrison Ford starring in two shows plus a trailer for the fifth installment of Indiana Jones.

And a switch to the news is likely to serve up Joe Biden, at 80 the oldest president in US history, or Mitch McConnell, the Republican leader in the Senate, who turns 81 on Monday. But while action heroes are evergreen, the political class is facing demands for generational change.

“America is not past our prime – it’s just that our politicians are past theirs,” Nikki Haley, 51, told a crowd of several hundred people in Charleston, South Carolina, as she launched her candidacy for president in 2024.

It was a shot across the bow of not only Biden but former US president Donald Trump, who leads most opinion polls for the Republican nomination but is 76 years old. Haley, notably, mentioned Trump’s name only once and avoided criticisms of him or his administration, in which she served as UN ambassador.

Instead, the former South Carolina governor called for a “new generation” of leaders and said she would support a “mandatory mental competency test for politicians over 75 years old”. It was a clue that in a party long shaped in Trump’s image, where ideological differences are likely to be slight, his senior status could offer primary election rivals a line of attack.

Lanhee Chen, a fellow at the Hoover Institution thinktank at Stanford University in Palo Alto, California, said: “She said what a lot of people are thinking, or are maybe afraid to say, and for that she deserves a lot of credit. The basic foundation of her argument, which is that we need to turn the page and find a new generation of leadership, is 100% right.”

Gerontocracy crept up on Washington slowly but inexorably. Biden, elected to the Senate in 1972, has been a public figure for half a century and, if re-elected as president, would be 86 at the end of his second term. At a recent commemorative event at the White House he hosted Bill Clinton, who was president three decades ago – but is four years his junior.

The octogenarian McConnell is the longest-serving leader in the history of the Senate and has offered no hint of retirement. Chuck Schumer, Democratic majority leader in the same chamber, is 72. Senator Bernie Sanders, standard bearer of the left in the past two Democratic primaries, is 81.

But there are finally signs of erosion in the grey wall. Last month Patrick Leahy, 82, a Democrat from Vermont, stepped down after 48 years in the Senate. Last week Senator Dianne Feinstein of California announced her retirement at 89 after months of difficult debate about her mental fitness.

Most profoundly, last month saw Democrats’ top three leaders in the House – Nancy Pelosi, 82, Steny Hoyer, 83, and 82-year-old Jim Clyburn – make way for a new generation in Hakeem Jeffries, 52, Katherine Clark, 59, and 43-year-old Peter Aguilar, as well as the arrival of Maxwell Frost, now 26, hailed as the first Gen Z congressman.

Presidential candidates on both sides of the aisle may now seek to harness this hunger for change in the contest for the world’s most stressful job in 2024. A CNBC All-America Economic Survey in December found that 70% of Americans do not want Biden to run for re-election, giving his age as the principal reason.

Chen, who ran unsuccessfully as a Republican candidate for California state controller last year, commented: “He has exhibited some of the manifestations of somebody who probably has seen better days and that’s hard to hide on the campaign trail. There’s a big difference between running for president at 70 or 75 – and what was possible in the 2020 election when Covid was still raging and a lot of the interactions were different – than running in 2024. I do think his age is going to be an issue.”

Biden typically brushes off such talk with the simple refrain: “Watch me.” The president underwent a routine medical checkup this week and Dr Kevin O’Connor, his personal physician since 2009, concluded that Biden “remains a healthy, vigorous 80-year-old male, who is fit to successfully execute the duties of the presidency”.

Karine Jean-Pierre, 48, the White House press secretary, said: “If you watch him, you’ll see that he has a grueling schedule that he keeps up with, that sometimes some of us are not able to keep up with.”

Noting Biden’s string of legislative achievements, she added: “It is surprising that we get this question when you look at this record of this president and what he has been able to do and deliver for the American people.”

After a strong performance in the midterm elections, a serious challenge to Biden from within the Democratic party still looks unlikely. Defenders say the obsession with his age merely illustrates his lack of other vulnerabilities after two years in which he has done much to win over moderates and progressives.

Tara Setmayer, a former Republican communications director on Capitol Hill, asked: “Did anybody watch the State of the Union? Joe Biden is fully capable of executing his job as president of the United States. He’s in better shape in some people half of his age. So they need to start focusing on the positives because repetition creates reality: perception is reality in politics.

“It’s a distraction and it undercuts the successes that Joe Biden actually has as president of the United States. There is much more concern over Donald Trump’s mental acuity and physical presence than Joe Biden. Joe Biden can run circles around Donald Trump.”

A White House doctor once memorably proclaimed that Trump has “incredible genes” and could have lived to 200 years old if only he had been on a better diet. But on the Republican side he could face challenges not only from Haley but Florida governor Ron DeSantis, 44, former secretary of state Mike Pompeo, 59, former vice-president Mike Pence, 63, and 57-year-old Senator Tim Scott.

Each has previously endorsed Trump’s “Make America great again” mantra and may now struggle to disavow it. No-holds-barred attacks on Trump himself risk alienating his fervent base. But as Haley showed this week, the promise of generational change might serve as a coded rebuke in party that is no stranger to dog whistles.

Drexel Heard, 36, who was the youngest executive director of the biggest Democratic party in the country (Los Angeles county), said: “Hypocrisy is a weird thing in American politics. It’s going to be interesting to see if Nikki Haley only talks about Joe Biden’s age and doesn’t talk about Donald Trump’s age and how the media calls her out on that. She’s going to say things like: ‘Well, you know, I’m just saying that we need generational change.’ She’s never going to call Donald Trump out.”

Trump will not be the first Republican candidate to face questions over his age. At a debate in 1984, the moderator reminded Ronald Reagan that he was already the oldest president in history at that time. Reagan, 73, replied: “I want you to know that also I will not make age an issue of this campaign. I am not going to exploit, for political purposes, my opponent’s youth and inexperience.” Even his Democratic opponent, Walter Mondale, laughed at the line. Reagan won re-election in a landslide.

Trump, for his part, will have an opportunity to silence Republican doubters at his raucous campaign rallies. Bill Galston, a former policy adviser to Clinton, said: “If he can’t do that, if he seems older and less energetic, then I can imagine the generational appeal sticking. But if his juices start flowing and he is able to do what he did seven years ago, then the generational appeal will be likely to fall somewhat flat.”

Galston, a senior fellow at the Brookings Institution thinktank in Washington, and a 77-year-old grandfather, added that it is not the “consensus view” among Republicans than Trump is too old to move back into the White House. “There’s a lot more support inside the Democratic party for the proposition that Biden is too old than there is inside the Republican party for the parallel proposition that Trump is too old,” he said.

Of all the Congresses since 1789, the current one has the second oldest Senate (average age 63.9) and third oldest House of Representatives (average age 57.5). Critics say the backup of talent puts it out of step with the American public, whose average age is 38. One example is around the tech sector and social media as members of Congress have often struggled to keep pace with rapid change and its implications for society.

Larry Sabato, director of the Center for Politics at the University of Virginia, said: “I’m 70, so I have great sympathy for these people: 80 is looking a lot younger than it used to, as far as I’m concerned. But no, it’s ridiculous. We’ve got to get back to electing people in their 50s and early 60s.”

“That’s the right time for president. You have a good chance of remaining reasonably healthy for eight years if you get a second term. Everybody knows that makes more sense but here we are. What can you say? This was the option we were given in 2020 and we’re going to get essentially the same one in 2024.”