Joe Biden is celebrating his 78th birthday on Friday, an age that will make him the oldest president ever to enter office when he is inaugurated on 20 January.

Biden’s age, which Donald Trump – four years his junior – sought to make an issue throughout the election campaign, including the nickname “Sleepy Joe”, is 23 years above an average 55 years of age for accession to the presidency.

Ronald Reagan, who also drew attention for his advancing years, was 69 when he entered office in 1981. Donald Trump was 70, making him the oldest person to be elected to the presidency – excluding Reagan’s re-election at 73.

The youngest elected president is John F Kennedy, who was 43 at the time of his inauguration and 46 at the time of his assassination on 22 November 1963, while the youngest to assume the presidency, after the assassination of William McKinley in 1901, was Theodore Roosevelt at 42.

As far as Biden’s physical health is concerned, his most recent publicly-issued medical assessment, released in December 2019, reported him as “healthy, vigorous ... fit to successfully execute the duties of the Presidency, to include those as chief executive, head of state and commander in chief.”

He was listed as 5ft11in tall, weighed 178 pounds, and had a blood pressure of 128/84 at that time. He had two brain aneurysms in 1988. In 2009, he had episodic atrial fibrillation. Biden does not smoke or drink alcohol and exercises five days a week, according to his physician.

Throughout his election campaign and into the current, contentious phase of transition, Biden’s handlers have taken steps to counter age-related misgivings – in part stemming from his light election campaign schedule know as the the “basement strategy” during the coronavirus pandemic. This included often having the now-president-elect jog to the podium at speaking engagements.

In an interview with CNN, Biden promised to be “totally transparent” about all facets of his health if elected. However, he hasn’t said how he’ll do that.

“It’s crucial that he and his staff put himself in the position early in his presidency where he can express what he wants with a crispness that’s not always been his strength,” said Ross Baker, a political scientist at Rutgers University who has advised legislators from both parties.

“He has got to build up credibility with the American people that he’s physically and mentally up to the job.”

According to Baker, Biden’s advanced age puts greater emphasis on the quality of his staff picks. Indeed by selecting his vice-president Kamala Harris, 20 years his junior, who emerged during the democratic nomination campaign as one of his fiercest critics, was a tacit acknowledgment of his age issue. Biden himself has described his role as “transitional” for the future of the Democratic party.