But recent polls have shown a shift among older voters towards Mr Biden, who held a nine-point lead against Trump with the group in a recent New York Times/Siena survey and a 12-point lead in a Quinnipiac University study last month.
Campaign advisers have suggested the trend is due to the president’s proposal to safeguard welfare benefits for retirees, as well as policies geared towards reducing the cost of prescription medication.
Biden insiders have also argued that their warnings about the threat Mr Trump poses to democratic institutions resonates more with older voters.
However, serious concerns for the White House remain, with a majority of Americans suggesting Mr Biden is too old to govern effectively in a second term.
A stark poll released on Thursday found half of the US voting public think the Democrat will forget where he is in his upcoming televised debate against Mr Trump.
Some 41 per cent also said they thought Mr Biden would walk the wrong way off the debate stage and 40 per cent predicted he will have problems standing up for the debate, according to the survey by JL Partners.
Mr Trump, who is three years Mr Biden’s junior, has trodden a delicate balance on the age issue, attacking the president’s mental acuity while insisting it is not age-related.
“If you go back 25 years, he wasn’t the sharpest tack either,” he said last year.
Warned against complacency
Democrats have warned the Biden campaign against complacency, with polls suggesting Mr Trump’s criminal conviction for falsifying business records has done little to move the race beyond a dead-heat.
Poll after poll has also shown Mr Biden’s winning 2020 voter coalition weakening, with support waning among young voters and ethnic minorities.
Dr Biden has become one of the White House’s most active campaign surrogates, with planned stops in the battleground states of Wisconsin, Nevada and Arizona, as well as Minnesota and California.
Mr Trump has not publicly marked his birthday. He suggested he would rather not spotlight the milestone during a rally in Las Vegas over the weekend.
“There’s a certain point where you don’t want to hear ‘happy birthday’. You just want to pretend the day doesn’t exist,” he said.