In film history, there’s never been a project like the nine “Up” British documentaries, which have presented unique challenges for director Michael Apted and editor Kim Horton as they follow the lives of British individuals in seven-year intervals. Horton, who has edited the films since the 1984 “28 Up,” says, “It’s probably the greatest thing I’ve ever done. Michael refers to it as his life’s work, and I see it the same way.”

The first film, “Seven Up!,” was made in 1964 for British TV, and intended as a one-off to show class differences among a group of 7-year-olds. Seven years later, someone suggested a follow-up, and it proved so successful that Apted has been directing new editions every seven years.

BritBox makes its theatrical debut with “63 Up,” currently in theaters before its launch on the streaming service. The new edition follows the same format as the others: Each subject is given about 10 minutes, during which Horton must present a recap of their lives from the film archives, along with updates via a one-on-one interview and B-roll footage.

“We treat each film as if a viewer hasn’t previously seen the others,” Horton says. “Some archive moments are familiar, because why wouldn’t you use those? But with each new interview come new opportunities. I like to go back into the archives to find stuff we haven’t used before.”

Horton says that many viewers are longtime fans. “With Twitter you get feedback from people, saying, ‘Why didn’t you use the dog chasing the rabbit when Suzy was 14?’” he says.

It’s impressive that of the original 14 participants, only one dropped out — interestingly, he’s become a documentary maker himself. For “56 Up,” 13 of the 14 participated, but the new edition is down to 12, due to the death of one woman, marking the first passing in the series.

And while a few “63 Up” interviewees mention Brexit and Uber, the filmmakers downplay current events. “We’re pointing our camera at ordinary lives,” says Horton, “the things all of us go through: the families, the relationships. Out of that comes the emotion; when they’re talking about a child or the death of a parent, it’s something all of us can understand.”

Horton marvels at how an editor’s work has changed due to technical advances, but the storytelling remains consistent. The new film runs 138 minutes, comparable with the 133 minutes of “28 Up,” when he began. That’s surprising, considering how much more material could be covered.

The filmmakers also have stuck with one version of each film, rather than extended cuts for various platforms.

“We could go much longer if there were financing for alternate versions, but it’s always run on a tight budget,” Horton says. Besides, “you can’t push an audience too hard” with massive running times.

Horton praises Apted and the “Up” team, and he’s grateful for the opportunity to be on it. “The emotion is what interests me as an editor. To have that material, it’s the best. You can’t get better than dealing with people’s lives.”