Joe Biden turns 80 on Sunday, making him the first octogenarian president in United States history.

He is set to celebrate his birthday with a brunch hosted by his wife, Jill, a celebration that got an extra lift this weekend with his granddaughter's wedding at the White House on Saturday.

With Biden already the oldest person to serve as president, the 2024 race for the White House is shaping up to be uncharted territory for the US. The nearly 250-year old democracy celebrates youth, but millions of Americans – including presidents – are now working well beyond the traditional retirement age of 65.

Ronald Reagan was 77 when he left the White House, but Biden would be 86 by the time a second four-year term ends, should he win it. His leading potential Republican opponent, Donald Trump, would be 82 when he left office if he won in 2024.

As a society, the US is aging, and working until later in life. The 65-and-older population is projected to nearly double from 52 million in 2018 to 95 million by 2060. By 2026, more than one in four men over 65 will still be working, the nonprofit Population Reference Bureau projects.

Some Americans have concerns about the advanced age of the two most likely 2024 candidates.

While 71 percent of Democrats think Biden is "mentally sharp and able to deal with challenges," 46 percent say he may not be up to the challenge of running in 2024, according to a Reuters/Ipsos poll taken Nov. 8 and 9. About a quarter of Republicans, 26 percent, think Trump may not be up to 2024 because of his age.

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