Many of them were children -- middle schoolers and high schoolers -- on that bright day in September 2001.
Some were a bit older, just entering college or serving in the military.
But all of them can remember where they were when they heard the news that planes had crashed into the World Trade Center, the Pentagon and a field in Pennsylvania.
Alli Barocas and John McAuliff were in middle school in New York City on Sept. 11, 2001. They recall teachers pulling some of their classmates into the hallway to tell them the building where their parents worked had been attacked. McAuliff says he doesn't like revisiting footage from that day -- it's too disturbing to relive.
Every generation has an event that binds them together in history, says educator and researcher Patricia Somers, who has been studying the effects of 9/11 on college students for nearly 10 years. For earlier generations, it was the Great Depression, the Kennedy assassination or the shuttle Challenger explosion. For so-called "Millennials" -- broadly defined as 18-to-29-year-olds -- the 9/11 attacks are considered to be that defining event.
"One way I like to look at it is these events shape a generation, and yet even more they reveal a generation," says historian and demographer Neil Howe.
The threat of another attack has made this generation hard to shock, says Daisy Alioto, 20, who attends Bowdoin College in Maine. Terrorism could happen any time, anywhere and there's nothing to be done about it.
"When the threat level is always orange, how do you live? You get acclimated to always living under threat," says Alioto.
What Alioto describes reflects this generation's longing for stability and security, according to Howe's research. These young people grew up in a time of fear around every airport visit, uncertain of when the next attack would occur. They have been raised to strive for many of the benefits their parents may have enjoyed -- a steady job with a 401(k), a pension, homeownership -- but that lifestyle has been rapidly disappearing amid continued economic troubles.
Howe says that this generation, more than any other, puts a lot of trust in the power of government, specifically the power to keep them safe.
Many also feel nostalgia for the sense of community that swept over the country in the weeks following 9/11. They don't remember any other time in their lives in which they saw their neighbors come together to give blood, collect supplies and really talk to each other.
"At the time it was nothing but unifying. It was a very patriotic time. I liked that part of it," remembers Alioto.
The youngest members of this age group, now in high school, don't know a time when the U.S. wasn't at war. In an interview, a group of students at Denver East High School all agreed that the wars have gone on too long.
They cite a strong support and respect for the troops, but their generation is also much less likely to serve in the armed forces, according to a study by Pew Research Center's Paul Taylor.
"Today, 2 percent of the males in this generation of 18- to 29-year-olds are military veterans. If you look at the [Generation] Xers, same stage of life, 6 percent [of males] were military veterans. The Boomers, same stage of life, 13 percent were military veterans. The Silent Generation, 24 percent. So a profound shift here in one of the classic pathways to adulthood," says his report.
Jayne Hollen, a student at the University of Colorado at Boulder, said she feels her generation is more isolationist as a result of 9/11. The problems that have been heaped on their shoulders are immense: two wars, a deepening recession and growing national debt, just to name a few. It's more than one generation can be expected to fix, she says.
But despite coming to adulthood in a tumultuous decade, young people are still optimistic and resilient, and they are more involved in their communities, says researcher Somers. A 2008 study by the Harvard Institute of Politics found that 59 percent of people 18 to 24 years old said they were interested in engaging in public service for their country, and are striving for a higher education.
Neshea McCabe says that when she was little she used to think being rich and famous was the most important thing. But growing up post-9/11, she now has a different goal.
"I want to be president someday," she says.