US President Joe Biden is turning 81 – a milestone likely to draw attention to his status as the oldest person to occupy the Oval Office, with opinion polls showing Americans are worried he is too old for the post he is seeking re-election to.

Biden has addressed those who worry that he is too old for the rigours of the White House with humour and an attempt to convince voters that his age and experience across a half-century in public life is an asset in tackling America’s problems.

“I know I’m 198 years old,” Biden joked back in June.

If re-elected, Biden would be 86 by the end of his second term in office.

Republican Ronald Reagan, who had the prior record as the oldest US president, ended his second four-year term at age 77 in 1989.

Donald Trump, the frontrunner for the Republican nomination to challenge Biden in the 2024 election, is 77.

In a mid-September Reuters/Ipsos poll, voters expressed concern about Biden’s age and his fitness for office.

Some 77 per cent of respondents, including 65 per cent of Democrats, said Biden was too old to be president. Just 39 per cent said Biden was mentally sharp enough for the presidency.

By comparison, 56 per cent of poll respondents said Trump was too old for the office, while 54 per cent said he was mentally sharp enough to handle the challenges of the presidency.

-Reuters