Obama and Bush\'s former Defense Secretary says he doesn\'t know if Biden would make an effective president\, says a president in their 70s is \'problematic\'

robert gatesFormer Secretary of Defense Robert Gates testifies to support retired General John Kelly before a Senate Homeland Security and Governmental Affairs Committee confirmation hearing on Kelly's nomination to be Secretary of the Department of Homeland Security on Capitol Hill in Washington, U.S., January 10, 2017.Reuters/Joshua Roberts

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."

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.

"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.

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