We cannot imagine John F. Kennedy on his 100th birthday. For all of us, he will always be a man in his 40s, exuding the vigor that became one of his trademark words, pronounced in his distinctively New England way.

He was a student of history whose rhetoric gloried in the future, challenge and change. He became an icon even though he was an iconoclast. He could be coldly realistic, but he preached idealism. He honored intellectuals but mistrusted abstract thinking and ideology. He promised greater affluence but also challenged complacency.

He was a fervent Cold Warrior whose most important triumphs came in the name of peace. He avoided nuclear holocaust during the Cuban missile crisis and negotiated a partial nuclear test-ban treaty with the Soviet Union. He took office with a muscular promise that the United States would "pay any price, bear any burden" in the battle for freedom. But five months before his death, he became a prophet of what would be called detente, describing peace as "the necessary, rational end of rational men."

There was also the contrast between the organizing slogans of his Democratic forebears and his own. Woodrow Wilson's "New Freedom" and Franklin D. Roosevelt's "New Deal" were rooted in broad but specific objectives. Kennedy's "New Frontier" was more journey than goal, more temperament and disposition than program or wish list. He was a restless figure in a restless time.

He was also one very canny politician who revolutionized presidential campaigns, as Thomas Oliphant and Curtis Wilkie describe in "The Road to Camelot," their new book on Kennedy's five-year quest for the White House. His dad's money certainly helped, but so did Kennedy's understanding of his need to play a long game outside the lines of convention.

It is fitting that we honored the 100th anniversary of JFK's birth on Memorial Day, which salutes those who died for their country. Kennedy and his moment in history were shaped by the experiences of the Greatest Generation of which he was a part. World War II veterans developed a deep confidence in the capacity of Americans to act in common.

Government's role in pushing back against the Depression and then achieving victory over the Nazis and Imperial Japan bolstered their view of the United States as an inspiring democratic nation that stood on the right side of history. The shared sacrifices of combat strengthened the claims of a civil rights movement that Kennedy at first reluctantly and then wholeheartedly endorsed. "We are confronted primarily with a moral issue," Kennedy declared on June 11, 1963. "It is as old as the Scriptures and is as clear as the American Constitution."

Later in the 1960s, many Americans came to see post-World War II patriotism as naive and unrealistic. Especially in the eyes of the Vietnam War's opponents, it could also be dangerously arrogant and self-serving.

Confidence in government was sapped, and a long era of debunking began. Kennedy's own reputation came in for some of that. His imperfections, long known to his friends and some of his enemies, became part of his legacy.

Yet Kennedy's heroic status has never faded, and not simply because he was taken from us on Nov. 22, 1963. It was an act of violence that presaged a decade that would grow more violent still.

Across our increasingly rigid political divides, we still treasure his cool and ironic perspective on life that coexisted with an infectious sense of public service. He drew thousands to government and tens of thousands to a Peace Corps that was one of his proudest creations.

And he is the president who is forever young.

"I think there attaches to him his youthfulness, his hope, his promise, and the country won't let that go," the historian Robert Dallek once told NPR. Dallek's fine Kennedy biography is affectingly titled "An Unfinished Life." All these years later, we share a collective sense of an unfinished era, an unfulfilled promise and a lost opportunity.

One of Kennedy's most admirable traits was his talent for maintaining a critical distance from himself and who he became. Theodore Sorensen, who was 24 when he first went to work for JFK, noted in 1965 that his old boss was "a constant critic" of his own myth.

So let's not mythologize Kennedy. But let us remember with respect and gratitude a man who scorned the yearning for "the comfort of opinion without the discomfort of thought." Kennedy's self-awareness, his dedication to discovery, and his success in marshaling our nation's energies in a more hopeful time are a gift to us still.

— E.J. Dionne's email address is ejdionne@washpost.com. Twitter: @EJDionne.