What’s the difference in age between the oldest president to assume office and the youngest?

Joe Biden has defeated Donald Trump and will become the oldest president in American history, a title previously held by Ronald Reagan.

President-elect Biden, who will turn 78 on Nov. 20, was born in 1942 during World War II.

The oldest sitting president in U.S. history was Ronald Reagan, who was 77 when he left office in 1989.

Biden will become the oldest president on Inauguration Day – Jan. 20, 2021.

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Last year, Biden addressed the issue of age on ABC's "The View."

"I think it's important for people – it's a legitimate question to ask about my age," Biden said. "Hopefully, I can demonstrate that ... with age comes wisdom and experience that can make things a lot better. That's for you all to decide, not for me to decide."

On December of 2019, Biden’s doctor released a report in which he said Biden is a "healthy, vigorous, 77-year-old male, who is fit to successfully execute the duties of the presidency."

The report, prepared by Kevin C. O'Connor – Biden's primary care doctor when he served as vice president alongside former President Barack Obama from 2009 to 2017 – noted that Biden has an irregular heartbeat. But it also said Biden has not required any medication or other treatment for the condition, known as nonvalvular atrial fibrillation.

Donald Trump, 74, was also the oldest man to assume the presidency. He was 70 on Inauguration Day – Jan. 20, 2017.

The other oldest presidents at the time of inauguration were Ronald Reagan, who was 69 in 1981; William Henry Harrison, who was 68 in 1841; and James Buchanan, who was 65 in 1857.

The youngest president was Theodore Roosevelt, who was 42 in 1901, when he began his first of two terms in the White House.

John F. Kennedy was the second-youngest president at 43, followed by Bill Clinton and Ulysses S. Grant, who were both 46 when they became president.

When the delegates to the 1787 Constitutional Convention pondered the question of what age a president should be, the big concern wasn’t about the office-holder being too elderly but too youthful.

“George Mason was the principal advocate for age requirements for elective federal office, and his views were inscribed into the Constitution—over the objections of James Wilson,” explains John Seery, the George Irving Thompson Memorial Professor of Government and Professor of Politics at Pomona College, and author of the book Too Young to Run. “Rather than making a positive case in favor of the superior wisdom and maturity of elders, Mason derided the ‘deficiency of young politicians’ whose political opinions at the age of 21 would be ‘too crude & erroneous to merit an influence on public measures.’

“A generational smear, not an argument, won the day.”

As a result, Article II in the U.S. Constitution specifies a minimum age—35—but doesn’t set a maximum. In many instances, that’s enabled voters to elect presidents in their sixties and even in their seventies, an age when many ordinary citizens have retired.

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To some observers, the lack of an age limit for the nation’s highest office heightens the risk of getting a president who isn’t up to the rigors of the job. “I'm concerned about age-related dementia, which the job can accelerate given the pressure of the office,” explains Gary J. Schmitt, a resident scholar in strategic studies at the American Enterprise Institute. “But I'm also concerned about the higher percentage of the chance of death while in office, meaning we will be voting for one candidate but getting someone else who we have not vetted as seriously.”

If President Franklin Roosevelt had died while Henry A. Wallace was vice president instead of Harry Truman, for example, “U.S. history would likely have taken a quite different turn,” Schmitt notes.

Even so, with a few exceptions, most elderly U.S. presidents seem to have been remarkably vigorous and capable.  Here’s a list of seven presidents who were the oldest when they left office.

Joe Biden

Joe Biden, the 46th President of the United States.

Joe Biden was 78 when he took the oath of office as the 46th president of the United States in January 2021. This earned him the spot as the oldest president in U.S. history. Taking office at a time when the COVID-19 pandemic was taking an especially deadly toll on America's elderly population, Biden acknowledged his age. As he told hosts on "The View" during his candidacy, “It’s a legitimate question to ask about my age. Hopefully, I can demonstrate not only with age has come wisdom and experience that can make things a lot better.” 

Ronald Reagan

President Ronald Reagan in 1980 in Los Angeles, California.

Harry Langdon/Getty Images

Born February 6, 1911, the nation’s 40th president was 77 years and 349 days old at the completion of his second term in January 1989. While campaigning in 1980, Reagan tried to put to rest questions about his age by pledging that he would resign if the White House physician ever detected signs of mental deterioration.

Once in office, Reagan proved to be remarkably resilient, and survived an assassination attempt in 1981, as well as surgery in 1985 to remove a cancerous polyp in his large intestine. Reagan always seemed the picture of robust health, in part because he exercised regularly with weights and enjoyed horseback riding and performing manual labor at his ranch in California. Reagan was able to brush aside concerns about age with humor, once joking during a 1984 debate that “I am not going to exploit, for political purposes, my opponent’s youth and inexperience.”

READ MORE: How Gorbachev and Reagan’s Friendship Helped Thaw the Cold War

Donald Trump

President Donald Trump in his first formal portrait after winning the presidential election, 2016.

David Hume Kennerly/Getty Images

Born June 14, 1946, Trump was 70 when he won an upset victory in the Electoral College over Hillary Clinton. By January 2021, the 45th President was 74 years and 200 days old. Trump reportedly avoided exercise other than golf because he considered it unhealthy. He said in a July 2020 interview that a test showed he could recall a sequence of five words on a test designed to spot cognitive decline.

Dwight Eisenhower

 President Eisenhower at his desk in the White House, 1956.

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Born Oct. 14, 1890, the 34th President was 70 years and 98 days old when he left the White House in January 1961. The World War II hero was a regular exerciser who only weighed seven pounds more than when he graduated from West Point, according to biographer Jean Edward Smith. Nevertheless, he nearly didn’t make it out of his first term.

In 1955, while on a vacation in Denver, Eisenhower awakened with chest pains. Initially, his doctor didn’t realize the seriousness of his condition, and hours passed before a cardiac specialist was summoned from a nearby military hospital to give him an electrocardiogram, which revealed that the then-64-year-old president had suffered a massive heart attack. Eisenhower had to spend six weeks recovering in the hospital, but despite his ill health, his popularity was so great that he easily won reelection the following fall.

READ MORE: How Gen. Eisenhower Spun a Humiliating WWII Defeat into Winning Military Strategy

Andrew Jackson

President Andrew Jackson.

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Born March 15, 1767, the 7th president was 69 years and 354 days when he completed his second term in March 1837. Though “Old Hickory” had a reputation for being a rugged ex-soldier and outdoorsman, by the time he reached the White House, he already had spent years coping with a variety of ailments.

According to biographer H.W. Brands, samples of his hair reveal that he had lead poisoning from old bullet wounds. Jackson also struggled with chronic diarrhea from diseases he’d contracted while fighting the Indians in the 1810s. His habits of smoking and chewing tobacco didn’t help his health either, and according to biographer Sean Wilentz, Jackson became so sick at times during his two terms that it appeared he might not survive.

Jackson did make it to the end of his term but when returned to the Hermitage, his plantation in Tennessee, the white-haired ex-president was physically spent and suffered from blinding headaches, insomnia, severe pains in his side and a chronic cough.

READ MORE: How Andrew Jackson Rode a Populist Wave to Become America's First 'Outsider' President

James Buchanan

James Buchanan, 15th President of the United States.

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Born April 23, 1791, the 15th president was 69 years and 315 days old when his single term in office ended in March 1861. Buchanan was 50 pounds overweight and his hair already had turned white by the time he took office in 1857, but his health deteriorated even more rapidly under the strain of the job, according to biographer Jean H. Baker.

Buchanan had trouble remembering orders he had given, and he became so physically and mentally drained that he was unable to get out of bed some days, and made his advisors come to his upstairs library at the White House to meet with him. He also suffered from hand tremors.

Given the health difficulties that he struggled with, it’s probably not surprising that he failed in his single term to heal the rift between the slave and free states that led to the Civil War.

READ MORE: Why Is James Buchanan Considered One of the Worst US Presidents?

Harry S. Truman

President Harry S. Truman, 1948.

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Born May 8, 1884, the 33rd president was 68 years and 257 days old when he left office in January 1953. Truman, who ascended to the presidency when Franklin D. Roosevelt died in 1945, was a diligent exerciser, who even in his sixties walked 1.5 miles each day at the same vigorous 120 steps-per-minute pace that he’d used while marching in the U.S. Army.

“He was in good shape,” William Seale, a historian and journal editor with the White House Historical Association, told CNN in 2016. But the strain of leading the nation through the brutal Korean War, and Truman’s habit of working 18-hour days and ignoring illnesses, almost got to him. In the summer of 1952, he became so sick that he had to be hospitalized, and doctors discovered that he was suffering simultaneously from three different bacterial infections. As an article from the National Archives website notes, the seriousness of his illness was kept from the public.

READ MORE: When Harry Truman Pushed for Universal Health Care

George H.W. Bush

President George H.W. Bush standing in the oval office, 1990.

Dirck Halstead/The LIFE Images Collection/Getty Images

Born June 12, 1924, the 41st president had reached 68 years and 222 days in age when he left office in January 1993. After a long career in government that included a stint as Central Intelligence Agency director and eight years as vice president, Bush had a lot of mileage on his tires the time he reached the Oval Office. But a lifetime of exercise had kept the former Yale University baseball star remarkably fit for a man in his sixties.

Bush was a regular runner who frequently invited reporters along on his runs, former White House correspondent Kevin Merida later recalled in a piece for sports website The Undefeated. Bush did have some stumbles that some interpreted as signs of being tired and out of touch, including a moment in which he checked his watch during a 1992 debate and then had difficulty answering an audience member’s question about how the recession had affected him.

Though he lost the election, historians have come to appreciate his achievements as president, including his handling of the end of the Cold War.

READ MORE: George H.W. Bush’s Role in WWII Was Among the Most Dangerous

Presidents Live Longer Than Most Men

It’s a common belief that the stress of being president tends to accelerate a person’s aging. But a 2011 study by S. Jay Olshansky, a professor in the School of Public Health at the University of Illinois at Chicago, found that U.S. presidents—at least the ones who weren’t killed by assassins—actually have tended to live longer than other American males who were their contemporaries.