Turning 18 is a milestone.

You’re officially an adult, maybe leaving home for the first time, for college or work. You can finally vote, buy a lotto ticket and serve in the military.

It’s a time of mixed emotions, made even more so if you’ve never really met the man who helped bring you into the world: the father you lost the day the towers fell.

For the widows of 9/11, some of whom were profiled by The Post on the 10th anniversary of the attacks, some had no choice but to put their grief aside in order to raise their families. As for the babies they were expecting and the newborns they cradled in their arms: Now turning 18, they’ve grown up dreaming of a parent they’ve only heard about.

“I believe he’s up there, helping me with my success,” says Allison Lee, born two days after her father, Dan, was killed in the terrorist attacks. Next month, she’ll move to Los Angeles, where her father grew up, to begin a dance program.

Allison says she can picture him giving her a thumbs-up, just as he’d done in the photos she’s seen. “I know he’d be telling me, ‘You’ve got this. Don’t give up on your dream.’”

Here’s where several 9/11 families are now.

He’s continuing the legacy

NYPD detective Joseph Vigiano had just three months with his infant son, John, before he ran into the World Trade Center on Sept. 11, trying to rescue the people trapped inside. Their father-son time was short, but precious.

“I have a picture I took of John sleeping on Joe [on] the couch,” says his wife, Kathy, who met Joseph when they were police officers stationed in Brooklyn’s 75th Precinct and is now retired from the force. “I guess that’s the best bonding you could do with an infant.”

With his older boys, Joseph Jr. and Jimmy, Joseph was a hands-on dad, says Kathy, now 54 and living on Long Island. He made his sons mini NYPD uniforms, cutting up one of his leather belts to fashion holsters that held flashlights and other tools.

“He was supposed to become their Boy Scout leader, but he died before the first meeting,” she says.

Kathy was at a school bus stop on Long Island with her three sons when a FedEx driver told her about the attacks. At first, she was glad to know her husband had responded. “He loved his job and he loved helping people,” she says. Days later, when she was called into police headquarters, she learned that his body was found in the rubble of Ground Zero. His brother, firefighter John Vigiano, was also killed.

Although Kathy’s youngest child grew up without knowing his father, he says he’s felt his presence all his life. “He’s looking out over me,” John, now 18, says. When he learned his dad started a lacrosse league with the NYPD, he took up the sport, playing it throughout his time at St. Anthony’s High School. Now, a Marine Transportation major at SUNY Maritime in the Bronx, John says he’s doing everything he can to make his dad proud.

That includes his aspirations for service: His late uncle, grandfather and great-grandfather were all FDNY firefighters. When John finishes at SUNY Maritime, he intends on applying to both the NYPD and the FDNY.

“[My dad] pursued the things that he was passionate about, and earned the respect of the people around him while doing it,” John says. “He would be proud of me looking to better myself.”

She found a way to connect

Early in September 2001, Dan Lee was preparing for one last business trip before his daughter’s birth. He and his wife, Kellie, had even picked out a name: Allison.

“I remember him talking to her through my belly before he left,” says Kellie, now 50.

In Boston on Sept. 11, Dan boarded American Airlines Flight 11 for the trip home to Los Angeles. It never arrived — diverted by terrorists into the north tower of the World Trade Center, killing all those aboard and, after a second plane hit the south tower, some 2,600 others in the towers and on the ground.

Allison was born on Sept. 13. In the couple of days that followed, the hospital placed Kellie on suicide watch.

“I just felt hollow,” says Kellie, who also had a 2-year-old daughter, Amanda, at home. “But I had babies to take care of.”

With time, the family began to heal. They moved to Las Vegas, where Kellie remarried, and her second husband, Chris, came to be like a father to the girls. Every Sept. 11, the family visits Dan’s favorite restaurant, Islands, which has an outpost in Vegas. And while her sister has vague memories of their father, it was different for Allison. “All you get are stories,” Allison says.

Still, she’s found a way to feel close to her dad, who was a drummer: She’s been dancing since she was 8.

“She got his long legs and his rhythm,” her mom says.

Allison, who’ll move to Los Angeles next month to study at the Millennium Dance Complex, says that dancing helps her deal with the loss that’s shaped her life.

“Anything I’m feeling, I can express through dance,” she says. In 2016, she and her dance troupe performed a tribute to Sept. 11, with her family’s story woven throughout the interpretation. It was an emotional experience, she says.

“Once we danced through it, I realized a whole part of me is missing,” she says. “It helped me process it.”

Most days, though, the strongest feeling she has when she thinks about Sept. 11 is one of gratitude, for her mom.

“I think about how strong she was to go through that and still do all these things for us,” Allison says. “She’s the most positive person I know.”

He still doesn’t know

There are signs that Joseph Reina is deeply connected to his late father, Joe.

Joe was an operations manager for Cantor Fitzgerald, working on the 101st floor of the north tower when the planes hit. His wife, Lisa, was almost 8 months pregnant. She gave birth to Joseph in a haze on Oct. 4, still dreaming, she says, that her husband would find his way home.

In the months and years to come, she believes that she and her child sensed Joe’s presence.

“When Joseph was a baby, he would look up at the ceiling and just smile,” says Lisa, now 49 and a retail worker from Staten Island. “I would always say, ‘Do you see Daddy?’ ”

Lisa still sees her son look up and smile, although she’s yet to tell him what happened to his father: Joseph, who’s on the autism spectrum and has difficulty communicating, wouldn’t be capable of comprehending the tragedy, Lisa says.

But she sometimes feels she doesn’t need to tell him. “He just has a feeling,” she says.

She saw the strong physical resemblance between Joseph and his father — “the kind of guy who could light up a room” — as early as her baby’s first Christmas, when she took his picture and saw his daddy’s funny smile. Back then, Lisa didn’t know how she was going to raise him alone.

“I wanted to crawl up in a ball and cry, but I couldn’t because I was thrown into that mother-father role automatically,” she says.

It was especially hard when Joseph, then 2, was found to have special needs. “Losing your husband is hard enough,” she thought. “How much more can I take?”

Lisa remarried in 2004 and had a daughter, now 15. Her new husband treated Joseph the same as he did his own child, and while the couple has since separated, her ex and her son are still close. She’s grateful that Joseph has a father figure in his life.

Joseph, who’ll soon enter a program for job training and daily living skills, is “the happiest kid you ever wanna meet,” Lisa says. He still hugs his mother, a gift she doesn’t take for granted.

Although he doesn’t know it, she says, it was Joseph who’s kept her going all these years.

“If he wasn’t here and I didn’t have him to have to take care of,” Lisa says. “I don’t know how I would have gotten through it.”