After more than a half century , the “Up” movies can be as hard to put your arms around as they are to resist. The sui generis documentary project — the latest and ninth installment is “ 63 Up ” — began in 1964 as a stand-alone production for Granada Television. The subject was class in Britain, as briefly seen through the lives of 7-year-olds from different families and socioeconomic circumstances. The 40-minute result could easily have been a didactic turnoff, but the black-and-white “Seven Up!” was a delightful, tender portrait of 14 lives in first bloom.

Remarkably, the filmmaker Michael Apted kept the project going and has revisited the original interviewees every seven years. (He started as a researcher on the first one, and has directed all the movies since.) Just as incredibly, most of the subjects agreed to continue the experiment with him, even as he pursued a career in features with dramas like “Coal Miner’s Daughter” and “Thunderheart.” In between fictional hits and misses, he has steadily returned to the “Up” series. As the decades have slipped away — as “21 Up” gave way to “49 Up” in an eye blink — he has revisited Tony, Sue and others , checking in with them as they grew up and grew older, developed acne and then potbellies, got married and had sweet children of their own.