Some of them have just started Grade 9. Some have grown so tall their mothers now look up at them. More than a few are learning to play guitar. And some will flash a mouthful of braces when they grin.

To a world that greeted them 14 years ago amid the worst U.S. tragedy since Pearl Harbor, they were known as the "9/11 babies." But they're babies no more.

The young adults born on Sept. 11, 2001, represent the promising new lives that arrived at a time when humanity was gripped by heartache, anger and confusion following a series of co-ordinated attacks on the U.S.

Today, on that tragic anniversary, an estimated 13,000 American teens will celebrate their 14th birthdays.

Among them will be Trevor Naman, Hillary O'Neill, Anish Shrivastava and Jake Tomlinson, a few of the young faces of 9/11 Day, a global campaign that promotes charitable engagement and good deeds to memorialize the goodness that pulled the world together following the Sept. 11 attacks.

Travelling to Manhattan

Trevor's mother, Christine, was already in labour in a Pennsylvania hospital that day 14 years ago as the hijacked American Airlines Flight 11 struck the North Tower of the World Trade Center shortly before 9 a.m. ET.

Trevor Naman

Trevor Naman holds up his own baby photo, in 'Faces of Hope' an anthology of photos of babies who were born on Sept. 11, 2001, that his mom published. (Matt Kwong)

Just an hour west of her hospital room in Monroeville, Penn., United Airlines Flight 93 crashed into a field in Shanksville, Penn. at 10:03 a.m.

Trevor was born at 2:07 p.m. Now a six-foot-tall hockey and football player with a firm handshake, he stands an inch shy of his father, Peter.

This year, his family travelled to Manhattan from Monroeville so Trevor and seven others sharing the same birth date can help ring the opening bell at the New York Stock Exchange, part of a kick-off event for 9/11 Day. (The kids will receive a surprise birthday cake.)

Along with Hillary and Anish, Trevor will also be volunteering with New York's City Harvest food rescue program.

"When people hear about my birthday, it's like, 'Wow!' They're amazed. But it's not really weird. I think it's a good thing to have a birthday on this day. It gives me a reason to be different," Trevor says. "9/11 Day is like about helping out the community."

'Turn the TV off'

As with Trevor's mother's experience, Heather O'Neill was going into contractions the morning of Sept. 11, 2001, as televisions in her Connecticut hospital room flickered images of the Twin Towers burning.

Hillary O'Neill

This summer, Hillary O'Neill set up a neighbourhood lemonade stand to raise funds for Al’s Angels, a local charity helping children with cancer. (Heather and Glenn O'Neill)

"It was probably around 11 a.m. when Heather finally requested they turn the TV off," her husband, Glenn, recounted.

Their daughter, Hillary, was born at 2:55 p.m.

A sunny Ninth Grader in Norwalk, Conn., Hillary loves to read and sing along to Vance Joy and Taylor Swift. She works as a camp counsellor.

This summer, she set up a neighbourhood lemonade stand to raise funds for Al's Angels, a local charity helping children with cancer.

"Being born on Sept. 11, it sort of made me live in the moment, because anything bad can happen," she says. "It's almost shaped who I am."

Those who learn about Hillary's birth date for the first time often react the same way.

"A lot of people, when they hear I was born on 9/11, it brings smiles to their faces because they realize that something very bad happened that day, but it was also a day when some good things happened."

Taking off 9/11 to visit a newborn

Anish Shrivastava

Anish Shrivastava, who turns 14 today, shares a birth date with an estimated 13,000 Americans. His uncle, who worked in the World Trade Center in 2001, decided not to go to work on Sept. 11 so he could meet his newborn nephew at a hospital in New Jersey. (Jaya Shrivastava)

One of the most remarkable stories was that of Anish Shrivastava, whose uncle worked inside the World Trade Center, but decided to take the day off on 9/11 so he could meet his newborn nephew.

Anish, a voracious reader, guitarist and Xbox gamer, was born at 10:05 a.m. in New Jersey.

"We always think that Anish came to save his uncle, and they share a very special bond," says Anish's mother, Jaya.

"And Anish just says, 'I really didn't have anything to do with it, mom, it was just by chance that I was born.'"

Jake Tomlinson was an unexpected surprise for his mother after her water broke more than three weeks early the night of Sept. 10, 2001.

By 12:23 p.m. the next afternoon in Jamestown, N.Y., Tammy Tomlinson was cradling her first-born in her arms.

"I just had a beautiful baby and was more excited about that than worrying about, oh gosh, what did I do, bringing this child into a world like this?" she says.

She later gave birth to three more boys. Jake, the oldest, wrestles and plays football. He's also more than happy to take on what his mother describes as the "let me irritate you" older brother role.

Jake Tomlinson

Jake Tomlinson will celebrate his 14th birthday by attending a homecoming football game and delivering baked goods to a local fire station as part of the 9/11 Day program. (Tammy Tomlinson)

"When he needs to be, he's a very good, very caring older brother," she adds.

Although Jake couldn't attend 9/11 Day today in Manhattan, he's celebrating his birthday at a homecoming football game and will bake brownies and cookies to deliver to the volunteer firefighter's hall in Bemus Point, N.Y.

"On a day like 9/11, you don't want to take away from those who lost loved ones, but there were lots of good things that happened that day, such as my son," Tammy says.

Symbolizing hope

That's a key takeaway for 9/11 Day, said David Paine, who co-founded the charity a few months after the 2001 attacks.

"The kids symbolize hope. At the very moment this terrible event was occurring and people were dying, there were all these children being born," Paine said. "They came to the world with hope and kindness and innocence. These children are representatives of the future, and they symbolize the side of goodness and how everybody came together on the day of that tragedy."

While the kids will be on a televised media tour in Manhattan with Fox and Friends, and possibly CNN, it's also important to have some perspective on the day, Hillary O'Neill's father, Glenn, says.

"One thing that kind of gets lost a little bit is, I kind of have to remember, hey, she's a 14-year-old girl and it's her birthday!" he said. "As much as Hillary is very excited to be a part of everything and help make a difference, at the end of a day, I think she'd like to get home, eat some cake and open some presents."

VIDEO: "Hope was Born on 9/11"