Washington (AFP) - Democratic former US vice president Joe Biden has conceded that his advanced years would be an issue if he ran for president in 2020.

At 75, Biden is three years older than Republican President Donald Trump and would be 77 by the time his party's primary contest got underway.

"I think age is a totally legitimate thing to raise," said Biden, who was vice president under Barack Obama, during a speech in Michigan on Tuesday.

"It's totally appropriate for people to look at me and say if I were to run for office again, 'Well God darn you're old,'" he said in remarks carried by CNN television. "Well, chronologically I am old."

Biden is often cited as a potential Democratic candidate who packs a solid resume and has some of the common touch needed to lure voters from Trump's populist movement.

Among other big Democrat names seen as potential candidates are populist leftist Bernie Sanders, 77, and media mogul Michael Bloomberg, who is 76.

Trump was the oldest American in history to begin a first presidential term when he entered the White House at age 70 years and 220 days in 2016. He says he is "100 percent" sure to seek reelection.