Will Joe Biden be a one-term president?

Joe Biden, the oldest person ever to win the US presidency, will celebrate his 78th birthday on Friday. 

If he were to run and be re-elected in 2024, he would be 86 at the end of a second term in 2029.

With two months to go before he even gets the keys to the Oval Office, Washington insiders are already asking: will he be a one-term president?

Throughout his campaign against Donald Trump, Biden -- a "lion of American history," according to his former boss, Barack Obama -- has remained purposefully vague about his future plans.

When asked by ABC News in August if the idea of serving for eight years was on his internal radar, Biden replied: "Absolutely."

But before that, in April, at a fundraising event, he told donors that he saw himself as a "transition candidate" -- a phrase that raised eyebrows and fuelled speculation.

Was he trying to say he was the best placed to shut the book on Trumpism, because of his decades of political experience and his empathetic nature, but would then pass the torch to a new generation of Democrats in 2024?

It goes without saying that many of the party's new, bright faces were not even alive when Biden was first elected to the US Senate in 1972.

Or was he simply talking about transition in a broader sense, without meaning to offer any outlook on the future?

A few days after securing the presidency over Trump, Biden's sister Valerie -- who has played a key role in his political career but generally remains out of the public eye -- expressed confidence he would seek re-election.

So, what did he mean by "transition candidate" then?

She told "Axios on HBO" that he was "transitional in that he's bringing in all these young people and bringing (us) back again (so) we're not a divided country."

Above all, one thing is clear -- Biden is trying to maintain a maximum of political capital going forward. 

No one can run for the White House and explicitly say it's for one term, after all. That would weaken his position and open the door -- too quickly and too widely -- to an all-out succession battle within the party.

'Sense of legitimacy'

For presidential historian Julian Zelizer, a professor at Princeton University, there is "no value" in Biden making his plans clear too early.

"In this age of polarization, you need to use every bit of muscle -- including the threat of re-election -- to move bills," Zelizer said.

In American history, the number of presidents who did not run for a second term is relatively low.

James Polk, who served from 1845 to 1849, campaigned on the fact that he would not run again -- and he kept his promise. But politics in the mid-19th century little resemble the circus in present-day Washington.

The only example in modern US history is Lyndon B. Johnson, who was catapulted into the White House in 1963 when John F. Kennedy was assassinated.

Johnson easily won his own term in the 1964 election against Republican Barry Goldwater, but in March 1968, with America weary of the Vietnam War and progressive Democrats challenging him, he said he would not run again.

For many observers, Johnson bowed out because he was facing certain defeat.

But his decision to leave the White House after six years in power was nevertheless a "Pearl Harbor in politics," in the words of a Democratic lawmaker from his native Texas.

Beyond a thirst for the power and prestige of the office, why are American leaders so hellbent on staying for eight years?

"The second term gives a president the sense of legitimacy," Zelizer said. "It's also a time to pursue difficult policy initiatives without electoral pressures."

'God darn, you're old'

Biden of course knows he's in a tricky spot.

In the fall of 2018, before he had even announced his third presidential run, he acknowledged to an audience at a Michigan speaking engagement that his age was a "totally legitimate thing to raise."

"I think it's totally appropriate for people to look at me and say if I were to run for office again, 'Well, God darn, you're old'," he said.

"Well, chronologically, I am old," he added, making it clear he believed that age was just a number and he is still full of energy and intellectually quick.

One thing is clear: when he takes office on January 20, Biden's Republican rivals -- and the pretenders to the throne in his own Democratic Party -- will be listening carefully to what he says on the topic.

They'll be waiting for the tiniest hint of a possible retirement from the man who, in November 2022, will become the first sitting president in his 80s in American history.