WASHINGTON (AFP) - President Donald Trump opened a fresh line of attack on Friday (April 26) against his leading White House rival Joe Biden, saying the 76-year-old Democrat is "making me look very young" and vibrant by comparison.

"I'm the youngest person. I am a young, vibrant man," a smiling Trump, age 72, told reporters at the White House when asked how old is too old to serve as commander in chief.

"I look at Joe, I don't know about him," he said of Biden, who launched his 2020 presidential bid Thursday with direct attacks on Trump's character and controversial performance in office.

"I would never say anyone is too old, but I know they're all making me look very young both in terms of age and I think in terms of energy."

When Trump was asked how he would beat Biden, whose working-class appeal could help him win back states like Michigan, Pennsylvania and Wisconsin that Trump snatched from Democrats in 2016, the president was succinct: "I think we beat him easily."

Trump is the oldest-ever US president, but he is three and a half years younger than Biden, who is leading the crowded Democratic field of candidates seeking the party's nomination next year.

The president is also five years younger than Bernie Sanders, the US senator who is second in polling for the Democratic nomination.

Of the 20 Democrats in the running, 10 are under age 50. Three of those are half Biden's age or younger.

Despite Trump's assertions of youthful vigour and health, he does have a weight problem.

In February, Trump was declared healthy by his doctor.

The lifelong real estate dealer has confounded health experts with his penchant for junk food and avoidance of strenuous exercise.

He weighs 110kg, his doctor said after an annual examination, which would put him in the category of mildly obese.