Would Bob Gates serve in a Trump administration?

Former Defense Secretary Bob Gates doubts that he could ever serve in a Donald Trump administration, but suggested it's not necessarily because of his views of the presumptive GOP nominee.

"I learned a long time ago never to say never, but let's just say that would be inconceivable to me," Gates said in an interview in Williamsburg, Virginia Friday with John Dickerson, host of CBS's "Face the Nation," which will air Sunday.

It's the kind of question he's answered before, albeit with Mr. Obama's name in place of Trump's. In his book, "Duty," Gates talked about how he dodged that question, and his answer will sound familiar.

"I devoted a fair amount of effort to quelling such speculation, often saying, 'I learned a long time ago never to say never, but the circumstances under which I would do that are inconceivable to me,'" he wrote. Gates served in the Obama administration for two-and-a-half years.

Another reason Gates may be reluctant to commit, however, is his age.

"Before the election, I will be 73, and let's just say I have stopped working on my resume," Gates, who worked in most presidential administrations since President Richard Nixon, told CBS on Friday.

Asked if he could work in a Trump administration if he were younger, Gates said, "I think there would have to be a conversation with the candidate."

For example, Gates said when then-President-elect Obama asked him to stay on as secretary of Defense, he said they had a "secret meeting" in the fire house at Reagan National Airport in Washington.

"I had sent him 10 questions, beginning with: Why do you think you can trust me? And what are your positions on Afghanistan and the defense budget and who is the rest of your team going to be," he said. "No matter what age I might be, those are questions that I think would have to be asked."

Gates will turn 73 years old in September.

Gates, who currently serves as the president of the Boy Scouts of America and chancellor at William & Mary, served as secretary of Defense between 2006 and 2011, serving for both Presidents George W. Bush and Obama. He also previously served 26 years in the CIA, including as director of the agency from 1991 to 1993.

He has not endorsed any of the presidential candidates remaining in the race, and it's unclear if he ever will. Gates, who's a Republican, told "Face the Nation" in a 2015 interview that Hillary Clinton was a "good secretary of State." They both served in the Obama administration together at the same time.

"I think she played a critical role in getting the much tougher sanctions on Iran...We agreed in terms of the Afghan surge. If anything, she was tougher than I was on that...I think that we certainly agreed in terms of how to deal with the very first phases of the Arab Spring," he told CBS last year.