For Max Cohen, a ninth grader at Penncrest High School in Media, the Twin Towers have always been down.

The nation has always been at war.

He's always felt the threat of terrorism.

And, for that matter, he's always had to take off his shoes at security checkpoints before getting on an airplane.

"It's kind of weird," the 14-year-old said, recalling a trip to New York to see the towers' footprint. "Like two ghosts standing there."

For Max and other teens who were born after or close to Sept. 11, 2001, the unspeakable horror of the terrorist attacks isn't a visceral, lived event, or even a terrible memory shared with the millions of Americans who watched on live TV. They occupy a flatter, less-impassioned space, the violence being something they learned about, not something they lived through.

"I don't really [feel] the emotional impact," said Lourdes Blazquez, a 15-year-old 10th grader who lives in Frankford. "I just think of the facts, and what you see in the movies and stuff."

Sunday, on the 15th anniversary, a generation rises for whom knowledge of 9/11 comes solely from images on the internet and television, conversations with family, friends and teachers, and pages in newspapers and history books.

"We talk about this all the time," said Clifford Chanin, vice president for education at the National September 11 Memorial & Museum in New York. "The kids don't have a memory, and they aren't as emotional as the adults expect that they would be."

Young people always struggle to conceive that the world around them was not always as it is now. Still, Chanin said, teenage visitors seem to implicitly grasp that today's society is somehow more fraught, defined by greater risk and less certainty.

"They want to understand: 'Is 9/11 where this comes from?' " Chanin said. " 'The wars that we're fighting? The issues around Muslims in America? Dimensions of service and responsibility?' "

The museum offers lesson plans for teachers at every level, acknowledging that the unforgettable is, for some, not a memory at all. This year Penguin Random House published What Were the Twin Towers? in its young readers history series, including the lost spires among topics such as the Great Depression, Pearl Harbor, Woodstock, and the Hindenburg.

Teachers can struggle with how and how much to teach about 9/11. Where does the story of the attacks begin? And where does it end, given its still-unfolding ramifications?

"We talk about terrorism a lot," said Christopher Johnson, principal of Science Leadership Academy at Beeber in Overbrook. "We also talk about, 'Why do other people in other countries not like us so much that they would do us harm?' "

Chelby Elam, a 15-year-old Science Leadership Academy student, knows what happened on Sept. 11, 2001. So do all her friends. The topic is unavoidable when the calendar turns to September. Some students know teachers who were in New York that day, and who now talk, or don't talk, about what they saw and experienced.

"Sometimes I do a lot of research on it, because I'm all into conspiracy theories," said the West Philadelphia teenager. "Fifteen years is a long time."

Of course, some young people thirst for information on the attacks, trying to integrate the events of that day into their conception of American society, teens such as Aden Savett, a 10th grader who studies advanced-placement U.S. history at Cherry Hill High School East.

No, he doesn't remember that day, he said, but that doesn't mean he can't feel its impact.

"The thing that stands out to me right now is how much it divided us," Savett said. "This division is exactly what the terrorists wanted. . . . It's taught me how easily fear can get the best of us."

Last year the conservative Young America's Foundation lugged a camera onto the campus of George Mason University in Virginia and asked, "Why was America attacked on 9/11?"

"I'm not 100 percent sure," a young woman responded.

"That's a good question," a young man answered. "I should know more but I don't."

And those were college students - people who dwell in an academic environment where the effect of past on present is a focus of learning.

"I don't think people are forgetting 9/11, but in some ways it's becoming blurred," said Edward T. O'Donnell, who teaches history at Holy Cross College in Massachusetts.

Even if young people know what occurred, it's hard to communicate the emotional shock, the compounding dread and fear of the anthrax mailings a week later, the anxious certainty that another attack was coming.

The piercing edge of emotion can dull, and over decades, people can forget.

In his book, Ship Ablaze, O'Donnell tells the story of the General Slocum, a steamship that burned on the East River near Manhattan in 1904, killing more than a thousand people and making headlines around the world. Until Sept. 11, 2001, the disaster ranked as the deadliest day in New York history.

But today the steamship tragedy has been largely forgotten, almost as if it never occurred.

On Sept. 11, 2001, Louise Cohen was pregnant with Max and, as the towers fell, wondering what type of world he would inherit. He's considered the same question.

"I didn't see it with my own eyes, but I can imagine it," Max said. "I think of the war on terrorism. And what resulted from that, and how it changed our country."

Does it seem strange that the country has always been at war all his life?

"It's pretty horrible to say, but I don't think about it that much, because it's always been there," he said. "Terrorism in general, France and Belgium and the [Orlando] nightclub, that all seems pretty usual to me. I've never known a world without all that stuff.

"When I talk to my parents, they say that didn't usually happen when they were younger."