Former Defense Secretary Robert Gates believes that age is a "problematic" issue in the 2020 presidential election.

In an interview with "Face the Nation," Gates said candidates in their late seventies — like former Vice President Joe Biden, who is 76, and Sen. Bernie Sanders, who is 77 — don't have the "kind of energy that I think is required to be president." Gates worked with Biden as defense secretary in the early years of the Obama administration.

"I'm not sure you have the intellectual acuity that you might have had in your 60s," said Gates, who is 75 and the chancellor of the College of William & Mary, where the interview took place. "The thought of taking on those responsibilities at this point in my life would be pretty daunting."

Gates, who identifies as a Republican, also said he believes President Trump, at 72, may also be too old for the job. Mr. Trump was the oldest president to take office when he was sworn in at age 70.

"It's a question people ought to address," Gates said. However, he noted that President Ronald Reagan was 73 when he was elected to a second term, and Reagan was "a pretty great president."

Nonetheless, Gates said that there is a question of mental acuity at a certain age, for himself as much as anyone else.

"When you're talking about being the president of the United States, the ability to do the job in every respect has always got to be a consideration," Gates said.

The nearly two dozen Democratic candidates in the presidential race range in age from 37 to 77. The two oldest candidates in the race — Biden and Sanders — are consistently at the top of national and regional polls.