There won’t likely be much fanfare at the White House about it, but President Biden on Sunday became the first 80-year-old president in America’s history.

Already in the record books as the oldest man — at 78 — to assume the office, his office on Sunday said only that the president would celebrate the occasion of his 80th birthday with a small family brunch at the White House.

In the coming months, Mr. Biden and his family will need to decide whether he wants to continue that record-setting streak and run for reelection two years from now. Should he succeed in winning a second term, he would be 86 at the end of this eight-year run.

Mr. Biden’s handlers and allies have all stated that he fully intends to run They  have been quietly making plans for him to do so, but Mr. Biden himself has at times seemed less adamant. “My intention is that I run again,” he said at a news conference this month. “But I’m a great respecter of fate.”

Many Democrats in Congress have, publicly at least, encouraged Mr. Biden to seek a second term. Speaking on ABC’s This Week Sunday, Congressman Adam Schiff of California echoed those sentiments. “I think he should” run, Mr. Schiff said. “I think he’s extremely capable. What he’s been able to do in the last two years is an unprecedented level of accomplishment, and if he wants to continue, I am for him.”

The decision last week of Nancy Pelosi, the 82-year-old House Speaker, to step aside in favor of a “new generation” of leadership in Congress, however, highlighted a growing uncertainty in the party he leads about the drag his age may present at the polls. In exit polls taken during this month’s midterm elections, 58 percent of voters told pollsters that the president does not have the mental capacity to serve effectively as president. Only 34 percent said he was a strong leader.

The presidency is hardly the only office dominated by elderly politicians. As Ms. Pelosi alluded to in her speech on the House floor, most of Congress’ current leaders — especially Democrats — are in their 70s and 80s, and the current Congress is the oldest the country has ever seen, with nearly one-fourth of its members over the age of 70.

Mr. Biden’s public appearances make it hard to deny that his age is catching up with him. His trademark trot has turned into a halting gait, and he frequently flubs words and phrases and at times appears lost or confused on stage.

During a trip to Asia this week, he bailed on a formal dinner with other world leaders without explanation and twice spoke of being in Colombia when he meant Cambodia. Supporters are said to wince frequently when he speaks, hoping he gets through his remarks without mistakes.

According to the Associated Press, Mr. Biden still begins his days with an 8 a.m. workout with a personal trainer and physical therapist when he doesn’t spend the time on a stationary bicycle. His aides, who are highly sensitive to questions about his age, say he often reads briefing books deep into the night and holds meetings well into the evening. He is rarely, however, up early, they said.

Mr. Biden is an outlier among presidents in terms of age. Before him, the oldest man to be elected to the highest office was President Trump, who was 70 years old when he was sworn in on January 20, 2017.  Before that, the record was held by Ronald Reagan, who was just shy of his 70th birthday when he took office in 1981.

Most American presidents have been in their 40s and 50s when they first took office. Besides Messsrs. Trump, Biden, and Reagan, only 10 presidents were in their 60s when they took office — John Adams, Andrew Jackson, William Henry Harrison, Zachary Taylor, James Buchanan, Harry Truman, Dwight Eisenhower, Gerald Ford, and George H.W. Bush.

The youngest person ever elected president was John Kennedy, who took office at the age of 43 and was 46 when he was assassinated on November 22, 1963. The youngest man ever to serve as president, however, was Theodore Roosevelt, who took over the presidency in the fall of 1901 after the assassination of William McKinley. Mr. Roosevelt would serve as president for eight years.