Now, Carter is 95. To help the low-key former chief executive celebrate his birthday on Tuesday, the Carter Center is inviting people all over the world to send him birthday messages.

Carter has reached yet another milestone despite thinking he was on the verge of death just a few years earlier, after doctors discovered that cancer had spread to his brain.

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Born on Oct. 1, 1924, in Plains, Ga., the typically cheery and humble former Navy lieutenant has had a life of service. Before taking office as president, he served as a state senator in Georgia and as governor.

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His presidency — remembered for the Camp David accords, the Iran hostage crisis and the creation of the Departments of Energy and Education — flamed out during the election of 1980, when Ronald Reagan defeated him.

After leaving the White House in 1981, Carter, then 56, and his wife, Rosalynn, returned to their hometown in southern Georgia. Yet another distinction: Carter has been a former American president longer than anyone.

He remains engaged in the causes and activities that matter most, the Carter Center’s director of communications, Deanna Congileo, told The Post earlier this year.

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He participates in Carter Center programs in global peace and health and annual Habitat for Humanity builds, and he received the Nobel Peace Prize in 2002, for work during and after his time in office. He also continues to teach Sunday school lessons and courses at Emory University.

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“Both President and Mrs. Carter are determined to use their influence for as long as they can to make the world a better place. Their tireless resolve and heart have helped to improve life for millions of the world’s poorest people,” Congileo wrote in an email to The Post in March, as he surpassed Bush’s record for longest-living president.

In 2015, Carter announced doctors had discovered a form of melanoma that spread to his brain.

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“I just thought I had a few weeks left, but I was surprisingly at ease,” Carter said at a news conference that August. “I’ve had an exciting and adventurous and gratifying existence.”

He received his first radiation treatment at 90. Four months later, he was — remarkably — cancer-free.

Bush, the 41st president, died on Nov. 30. Carter attended the funeral in Washington, along with President Trump and former presidents Barack Obama and Bill Clinton and, of course, Bush’s son George W.

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Carter says he takes pride in having “always told the truth.” He also says he is proud that former secretary of state Hillary Clinton and Sen. Bernie Sanders (I-Vt.) “comported themselves with dignity, talked about issues that matter and presented a vision for our nation,” during the 2016 presidential election. Still, thinking back on more than 40 years, Carter told The Post last year that he “regrets not doing more to unify the Democratic Party.”

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His 2016 message to the younger generation of voters was: “Stay engaged. Stay involved. And be sure to vote.”

Last month, at a Carter Center event, the former president, who voted for Sanders in Georgia’s 2016 Democratic primary, said he is keeping “an open mind” about the 2020 Democratic candidates.

“One of the major factors I will have in my mind is who can beat Trump,” he said.

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At that same event, Carter was asked what it would take for him to run for president again. The questioner noted that Grover Cleveland had served two nonconsecutive terms in the late 19th century.

“I hope there’s an age limit,” Carter said with a big grin as he sat next to the former first lady.

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“You know, if I were just 80 years old, if I was 15 years younger, I don’t believe I could undertake the duties that I experienced when I was president,” he said. “For one thing, you have to be very flexible with your mind. You have to be able to go from one subject to another and concentrate on each one adequately and then put them all together in a comprehensive way.”

Carter added that presidents “also have to adopt new ideas.”