We cannot get past it, we Americans. Not a half century later. Maybe not even ever.

The president with the easy grin in whom so much hope was invested. His wife, forever frozen in pink and pillbox hat. The motorcade. The sunny day. The shadowy man in the window with a rifle. Even more shadowy, the man on the nearby "grassy knoll" who perhaps existed, perhaps didn't. The flickering, silent color film of a leader's final moments. And the way it is described, even now, by so many Americans: the "loss of innocence" that left us vulnerable to so much of the heartache and tumult that was still to come. If, that is, we were ever truly innocent in the first place.

We should move on, maybe. But we don't. From that moment in Dallas - that moment scoured and buffed for so long, visited and revisited by so many people with so many agendas for so many years - from that moment until now, Americans will not let go of this event that changed so much and, just as significantly, was thought to have changed so much more. Even as the world lurched forward, Pause was pressed on that moment, and Play has never really been pressed again.

Why? Here's one two-word answer: baby boomers.

It is they who have carried this torch, they who have fueled its flame. When talk turns to the inevitable question - "Where were you when you heard the president had been shot?" - the dominant answer in American culture is this one: "I was in school." It is almost as if no adults were around on the Friday of the assassination, except as bit players. This is because baby boomers - who were, indeed, in school that day - are the ones who have shaped the national memories of this event.

For this generation - the Americans who were 17 and under on that day and, today, are from 67 down to, say, 49 - the assassination of John F. Kennedy remains the watershed event that birthed the decade we know as the '60s and rippled out, year after year, into politics and science and art and culture. It has been a singular snowball rolling down a hill, still gathering debris and holding onto momentum as it hurtles through succeeding generations.

"This murder in broad daylight ... Everything changed," says Oliver Stone, the boomer director who served in Vietnam and made a movie about it before turning his distinctively critical lens on the Kennedy assassination.

Because he knows what becomes clearer with each passing year: For better and for worse, it was the event that defined the generation that has defined the way we look at the world today.

"So, what constitutes a Baby Boomer? Opinions vary, but most agree that a Boomer was born between 1946 and the start of the Vietnam War (about 1963). I, however, submit that a real Boomer is defined by the recollection of a world-changing event: JFK's assassination." - Ron Enderland, operator of a blog called "I Remember JFK: A Baby Boomer's Pleasant Reminiscing Spot"

To define the collective traits of a generation - to broad-brush millions of Americans with a statement like "they think" or "they believe" - is a futile pursuit. Many have tried, particularly with the boomers. Most have fallen short. The group is too diverse.

Yet when it comes to this event, boomers are often united by the way they characterize it. Many have described it not simply as an ending - of a life, a presidency, an era - but as a beginning. It is variously cast as the start of when America took a turn for the worse, the beginning of deep distrust of government, the unleasher of many kinds of chaos - and, of course, the dawn of the acceleration of the Vietnam War and the out-of-control decade it defined.

"It has become a founding crisis ... for this generation in particular," says Art Simon, author of "Dangerous Knowledge: The JFK Assassination in Art and Film."

"They have made it their own," he says. "They made it part of what came after it. They made it part of this revision, or this crisis, over governmental legitimacy. Or they used it as a founding moment for the unraveling of government legitimacy."

Many who have chronicled the generation characterize the assassination in similar fashion. The sense that emerges is that many boomers found themselves in the curious position of being young enough to both experience it and not experience it - for it to be both a real event and something dreamlike, a fable that they saw through the eyes of parents, teachers, television before they were able to process it. The processing came later, as they grew.

In 1980, when the oldest boomers were 34 and the youngest barely 16, Landon Y. Jones wrote what remains one of the generation's definitive histories - a book called "Great Expectations." In it, the man credited with coining the term "baby boomer" wrote this of JFK's death: "For the Baby Boom children, this was the most mesmerizing moment of their youth. Time was frozen."

And in 1987, Todd Gitlin said this in his history of the era, "The Sixties: Years of Hope, Days of Rage": "There are times when an entire culture takes the shape of a single event, like rows of iron filings lined up by the force of a magnet. ... The educated young felt his call, projected their ideals onto him. His murder was felt as the implosion of plenitude, the tragedy of innocence. From the zeitgeist fantasy that everything was possible, it wasn't hard to flip over and conclude that nothing was."

This is the thing about seminal events, particularly ones that retain an aura of mystery and conspiracy: They become empty vessels to fill. And the Kennedy assassination, perhaps more than any event in our lifetimes except for 9/11, is the ultimate empty vessel for the media age that Kennedy himself helped create. The assassination's mythology acts as its own echo chamber: Each time it appears to recede into the distance (and there have been many), a fresh echo always manages to reverberate.

That this emerged from the Kennedy years is no coincidence. His was the first American presidency to incorporate real-time mythmaking into its central narrative, and the media followed suit, building the aura of a "Camelot" whose violent loss was all the more painful because of the storyline that had enveloped it. Those same forces were harnessed immediately upon his death, both by Kennedy's inner circle and the American media at large.

Thus, the millions of Americans who remember that era through childhood's looking glass were assisted by the multiple souvenir editions of Life and Look magazines and commemorative newspaper sections purchased by their parents. Today, more than 1,000 of these talismans are for sale on eBay.

To talk to members of this generation, to read what they've written and listen to what they've said about the assassination over the years, is to see a few major themes emerge. Mournfulness and mythmaking - the sense of something lost - are among the more obvious. But there are others.

There is the persistent insistence of government conspiracy, of events that have been hidden from the public - and of complicity by just about everyone for just about every reason. In the era of NSA spying, this remains as potent a notion as ever. If indeed there were and are actual conspirators, they would have found it easy, in the past 50 years, to hide among the vast crowd implicated in various versions of the conspiracy. This is not to say that the conspiracy theorists - who bristle at the term - are wrong, only that they can't all be right.

There is the sense that Vietnam might not have happened - or that this central trauma of modern America might have played out differently - had Kennedy lived. There is evidence to support this and refute it. "He was truly a man who was working toward peace ... He is not just a missing president," says Stone.

And lurking behind it all, there is a feeling that the assassination's mystery, the uncertainty about dark forces possibly at play, has somehow seeped into everything: If "they" can do THAT, the thinking goes, then they can do just about anything.

Lisa Pease, a researcher who has studied 1960s assassinations extensively, encapsulated this sentiment at an assassination symposium in Pittsburgh last month: "We have to know the truth of our past and our present," she said, "in order to make good decisions about our future."