Carter, who earlier this year became the longest-lived chief executive in American history, didn't tie his comments to any of his fellow Democrats running for president in 2020, but two leading candidates, Joe Biden and Bernie Sanders, would turn 80 during their terms if elected.

Biden is 76. Sanders is 78.

"I hope there's an age limit," Carter said with a laugh as he answered audience questions on Tuesday during his annual report at the Carter Center in Atlanta. "If I were just 80 years old, if I was 15 years younger, I don't believe I could undertake the duties I experienced when I was president."

Carter's observation came in response to a jovial inquiry about whether he had considered running in 2020 since he's still constitutionally allowed another term. The 39th president left office in 1981 at the age of 56 after losing his reelection bid to Ronald Reagan, who served two terms and left office as the oldest sitting president in history, at 77.

Either Biden or Sanders would be older upon their inauguration than Reagan was on his final day in the Oval Office. At 73, President Donald Trump is a record setter, as well. He eclipsed Reagan's mark as the oldest newly elected president in history and would become the oldest president to be reelected. Age has been a flashpoint for some critics of Trump, Sanders and Biden.

Carter, who turns 95 on Oct. 1, said the Oval Office requires a president "to be very flexible with your mind," particularly on foreign affairs. He was speaking on the 41st anniversary of the Camp David Accords, a peace agreement he negotiated with Egyptian President Anwar Sadat and Israeli Prime Minister Menachem Begin.

"You have to be able to go from one subject to another and concentrate on each one adequately and then put them together in a comprehensive way, like I did between Begin and Sadat with the peace agreement," Carter said.

"The things I faced in foreign affairs, I don't think I could undertake them at 80 years old," he continued, before adding with a smile: "At 95, it's out of the question. I'm having a hard time walking."

Carter said he remains undecided in the 2020 primary.

"I'm going to keep an open mind," he said, explaining that he wants to vote for a candidate who pledges to make the U.S. the world's leading champion for peace, human rights and equality. "One of the major factors I will have in my mind is who can beat Trump," he added, noting that he'll vote for the Democratic nominee in the general election regardless.

Still, Carter's assessments on age could leave him with few easy choices in the primary.

Carter repeated his previous disclosure that he voted for Sanders over Hillary Clinton in 2016, siding with the democratic socialist over the party establishment favorite. But Carter has since warned Democrats not to go too far left, lest they risk alienating independents and moderate Republicans who can help the party defeat Trump.

He has specifically cited proposals like a single-payer health insurance system as potential deal-breakers for some voters inclined to vote against Trump. Sanders and another leading progressive candidate, Massachusetts Sen. Elizabeth Warren, back single-payer health insurance run by the federal government. Warren is 70 years old.

Meanwhile, Biden is leading most national and early state primary polls in part because of his strength among more moderate Democrats. Other moderates in the field trail far behind Biden, Sanders and Warren.

When Carter ran and won in 1976, he was the outsider toppling establishment favorites. But the former Georgia governor also represented the more moderate wing of a party that had been dominated by Northeastern liberals.

Since his defeat, however, Republicans have used Carter as a liberal caricature. And Carter himself, through his work at the Carter Center, has embraced the role of an outspoken human rights advocate willing to criticize the world's establishment institutions and accepted world order.

He's long blasted Israel's treatment of Palestinians, even as both major U.S. parties more carefully navigated the U.S. alliance with Israel. As Israel tallies votes from its Tuesday elections, Carter lamented that returning hard-line Prime Minister Benjamin Netanyahu to power could "end the peace process" altogether. Exit polls show that Netanyahu's party fell short of securing a parliamentary majority, potentially threatening his position.

Speaking about his post-presidency legacy, Carter said he wants the Carter Center, which has focused since 1982 on public health and election monitoring, to be more willing to criticize the U.S. government, advocate for policies to combat the climate crisis and explicitly take sides against war.

"The Carter Center has been basically mute on the subject of global warming," Carter said, putting blame on himself.

He also warned Americans against the consequences of perpetual military conflict. He noted that China, the major economic and geopolitical competitor to the U.S., has spent four decades at peace since Carter normalized relations with Beijing. In that time, China has spent trillions of dollars on infrastructure and education, Carter said, while the U.S. has spent corresponding amounts on military engagement.

"That just shows you the difference between peace and war," Carter said, later adding, "I just want to keep the world at peace."