New York ? When the “Harry Potter” film series is completed, its three young stars — Daniel Radcliffe, Emma Watson and Rupert Grint — will have inhabited J.K. Rowling’s universe for half their lives.

Radcliffe, who is now 19, was 11 when he was cast as the boy wizard for the series’ 2001 debut. Watson, now 19, was 10 when she auditioned for the whip-smart Hermione Granger. Grint, the eldest of the trio, is 20.

“I’ve probably been Ron as long as I’ve been Rupert,” says Grint, who plays Ron Weasley, the ginger-haired, perpetually hungry friend of Harry and Hermione.

The cast and crew have taken a break from filming Rowling’s last “Potter” book — to be spread out in two films — to publicize the series’ sixth installment, “Harry Potter and the Half-Blood Prince,” which arrives in theaters Wednesday.

Early reviews of the movie, the second one directed by David Yates, have been positive; both Variety and The Associated Press suggested it was the best “Potter” film yet. The movies have become progressively more complex, darker and realistic — even amid the fantasy world of Hogwarts School of Witchcraft and Wizardry.

As the films have matured, so has the cast.

More so than any other installment, “The Half-Blood Prince” shows that Radcliffe, Watson and Grint have gone from children to young adults. With the end of the series and a sense of graduation looming, its young stars appear to have emerged from the most treacherous of adventures — child actor stardom — as remarkably grounded people and increasingly talented actors.

Looking back on early films

To watch the first “Potter” film is to be reminded how young the actors were when they began.

“For me to look back on the old films is an almost entirely destructive thing to do,” Radcliffe says. “I just torture myself over it. I mean, I was young. I can’t be held accountable for the performance I gave in the first two films: I was 11 and 12. I wasn’t like Dakota Fanning … who could seemingly just do it. It was very much a child’s performance.”

The many lauded Brit actors of the “Potter” films have influenced Radcliffe — perhaps none more than Gary Oldman, who played Sirius Black in several of the films, most notably the third, 2004’s “Harry Potter and the Prisoner of Azkaban.” Interestingly, Radcliffe pegs that film as the moment he realized he loved acting.

“Something happened at the age of 14,” he says. “I started taking it more seriously, which meant I started having more fun.”

Since then, his progress has been apparent with each new “Potter” film — “a biannual review,” Radcliffe calls it. He has begun moving away from Harry Potter, including a hilarious cameo in Ricky Gervais’ TV series “Extras,” and a well-reviewed performance in a revival of Peter Shaffer’s “Equus,” which ran in London in 2007 and on Broadway in 2008. Radcliffe played a deranged stable boy who completely disrobes — a scene much written about by the media.

Radcliffe counts his last year as both his “biggest leap” and an “overwhelming blitzkrieg of camera flashes.”

Recreating herself

Watson has a hard time recalling the beginning.

“This all happened to me so young,” she says. “It’s very hard to go back to that time and be like, ‘Did I want to do this?’ It feels very foggy.”

Watson has acted in a few other films, but she has spent most of her spare time throughout “Potter” — and this is very Hermione-like — studying. This fall, she’ll attend Brown University, says producer David Heyman. (Watson isn’t discussing her plans publicly.)

She expects to continue acting, but says college felt like the obvious decision.

“The three of us have been working solidly since we were 10 years old,” she says. “I just need a little bit of normality for a while, just a little bit of space to work out what I want and who I am — all the usual stuff. It’s just something I always wanted to do.”

She plans to study literature and art, but she has also shown interest in fashion. She signed to a modeling agency about two years ago.

“Fashion’s great because you’re able to re-create yourself whenever you want,” Watson says. “Dan had time to go away and do ‘Equus’ on Broadway and break out of ‘Harry Potter’ a bit, and I was always studying. So my way of getting casting directors to look at me in a slightly different way was modeling.”

An awkward adjustment

The bemused Grint — who “Azkaban” director Alfonso Cuaron once said was the one most likely to become a star — remains clearly grounded, even if he’s used his earnings to purchase a hovercraft. That playfulness is perhaps an essential quality to Grint, who was never inclined to view acting as a job.

“I don’t think I ever really made the connection of it being a career,” he says. “It was just something that was fun to do. In the early ones, I don’t think I took the acting too seriously. I just read the lines and got on with it. Over the years, you start to take it more seriously with different directors coming in.”

He says he’s enjoyed the “more adult” roles and feels more comfortable in front of the camera after an awkward adjustment: “This is something I was kind of thrown into,” he says.

Cast camaraderie

It’s clearly a strange ride for the trio, who have only a vague sense of how this all began for themselves. Though they don’t generally socialize offset, the camaraderie of going through it together has clearly helped.

“To have someone that’s in the same boat as you is a relief,” Watson says. “I wish in a way that there had been a fourth in the trio that was perhaps a girl, but they’ve been pretty great.”

All three are certain of one thing: When they wrap the last “Harry Potter” scene, there will be tears. Their adolescence is forever intertwined with the movies.

“These are some of the most important years of my life and I won’t be able to look back on any frame of this film without it being linked to a dozen memories,” Radcliffe says.