Former Defense Secretary Robert Gates said in an interview that aired Sunday that he didn't know if his former colleague, Joe Biden, could make an effective president.

Gates, who served in both the Obama and Bush administrations, has previously said Biden has "been wrong on nearly every foreign policy and national security issue for the past four decades."

Gates also said it was "problematic" to have a president in their 70s. Biden is 75, Sen. Bernie Sanders is 77, and President Donald Trump is 72.

He added that a president in their 70s doesn't have the "intellectual acuity" they may have had in their 60s.

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Former Defense Secretary Robert Gates levied a number of criticisms against his former Obama Administration colleague, Joe Biden, in an interview that aired Sunday, even saying he didn't know if Biden could be an effective president.

CBS News' Margaret Brennan asked Gates in the interview about previous comments Gates had made of Biden in his 2014 memoir.

"Joe Biden is impossible not to like," Gates' memoir said. "He's a man of integrity, incapable of hiding what he really thinks, and one of those rare people you know you can turn to for help in a personal crisis. Still, I think he's been wrong on nearly every foreign policy and national security issue for the past four decades."

When asked by Brennan whether Biden would make an effective president, Gates simply replied, "I don't know."

—Face The Nation (@FaceTheNation) May 12, 2019

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Gates, who served in both the Bush and Obama administrations, said he stood behind his 2014 remarks about Biden and was particularly concerned with how Biden would fare with senior military officials.

"He and I agreed on some key issues in the Obama administration, we disagreed significantly on Afghanistan and some other issues," Gates said. "I think that the vice president had some issues with the military. So how he would get along with the senior military and what that relationship would be, I think it would depend on the personalities at the time."

But Gates also took issue with Biden's age — as well as the ages of his fellow 2020 presidential candidate Bernie Sanders, who is 77, and the current president, who is 72.

—Face The Nation (@FaceTheNation) May 12, 2019

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"Having a president who is somebody our age — or older, in the case of Sen. Sanders — I think it's problematic," Gates said. "I think that you don't have the kind of energy that I think is required to be president. I'm not sure you have the intellectual acuity that you might have had in your 60s. It's just a personal view."

Gates, who is himself 75, added that he couldn't imagine shouldering the burden himself.

"For me, the thought of taking on those responsibilities at this point in my life would be pretty daunting," he said.