Seven Up! first hit the screens in May 1964, and was intended as a one-off programme, a snapshot of the British class system and the way it conditioned so much of life. Tim Hewat, the Australian founding editor of World in Action, had the idea for the programme, and took as its starting point the Jesuit saying: "Give me the child until he is seven and I will show you the man." The programme makers went looking for posh kids and poor kids, expecting them to play their allotted roles in life.

The Canadian Paul Almond directed that original 40-minute film, and a young Cambridge graduate called Michael Apted was one of the researchers, responsible for finding suitable children in the south of England while Gordon McDougall went in pursuit of children in the north. They found 20, of whom 14 – bright, spiky, funny – made the cut. The anonymous half-dozen can be seen frolicking at London zoo in that first programme, and you wonder how they feel having not been tracked for the rest of their lives. Relieved, perhaps.

Four of the 14 came from the East End of London, and two other boys were in care – these were the poor end of the spectrum. Contrasted with these were three posh boys from the same prep school, and a boy and a girl at other boarding schools. Somewhere in the middle were two boys from a suburban school in Liverpool, and one from the Yorkshire Dales. As Apted now admits, there were too few women – outnumbered 10 to four by the men – and the middle class was under-represented. The choices reflected the world as it was in 1964, when women's place was still in the home and society was split between employers and employees, captains of industry and shop stewards, the professions and the rest.

It is worth replaying the beginnings of what has become a television institution because, though it looks to have been inevitable that the makers would return every seven years to check on the progress of their proteges, it wasn't. They only returned at 14 because Denis Forman, the visionary head of Granada TV, suggested it, and Apted, whose career is now intertwined with the series, took over as director because Almond was making feature films in Canada. Even the name might have been different. Seven Up! was Granada founding chairman Sidney Bernstein's idea, and Hewat and Almond hated it because of possible confusion with the soft drink. Half a century later, the TV brand has become equally powerful.

We have now reached 56 Up and, though several of the participants have occasionally sat out the seven-yearly update, only one, TV producer Charles Furneaux, has made what appears to be a definitive break, even attempting to get Apted to excise his photograph from all material relating to the programme. The rest, who receive a not very substantial fee for the periodic intrusion on their lives, have stuck with it.

How long will it continue? "As long as I'm above ground, I'll carry on," says 71-year-old Apted when we meet on the 20th floor of ITV's London HQ. "Maybe if I wasn't above ground, someone else would take it over. Having come so far it seems a pity to just unilaterally stop it unless there is good reason. I've only ever said we'd stop if too many of them pulled out, or people didn't want to watch it any more. But neither of those things has happened." Producer Claire Lewis, who joined at 28 Up, wonders what will happen when the participants start to die. "When we lose somebody it'll make the others think very hard about doing it again. I don't know what effect that would have on us and on them. It's very hard watching yourself grow old on screen."

Lewis stays in touch with the participants between the visits, and says they have become a surrogate family. "It sounds cliched, but it's true. They have become like cousins, or brothers and sisters. It's an extraordinary relationship, and because we know them so well it's like seeing a member of the family that you haven't seen for three months or a year, and you pick up exactly where you left off." Apted accepts the family analogy, but says that like most families it can have its problems. "Some of us are close; some of us aren't close. Some of them like me; some of them don't. A family is a very good image of what this is, because we've been together for almost 50 years now."

Does he feel responsible for them after so long? "You're fearful when you go back that bad things will have happened. That's always the documentarian's curse. You want only good things to happen to everybody, but also you like good stories. We want a rich programme and we know that life has its ups and downs, but we are very close to them and it is painful if things aren't going very well."

The starkest example of things not going well was Neil Hughes, one of the Liverpudlians. He went from bright-eyed schoolboy to homeless drifter, and spent much of his 20s and 30s scratching a living in London and the Shetlands. By 49 Up he had found a sort of peace in Cumbria, was active in local politics and in 2010 stood for the Lib Dems in Carlisle. His is the strangest and most extreme of the lives portrayed in the series.

The new series, which starts on 14 May, will be in three parts, but even that is only a couple of hours of television. The series gives a snapshot of complex lives without following every twist and turn. That may be both a strength and a weakness. The audience is left wanting more, but equally some questions are left hanging. "You can't sum their lives up in 10 minutes," says Lewis. "All you can do is try with integrity to give a flavour of the kind of people they are and what's happened to them."

Do they tell the truth? "Some of them are incredibly open with us," says Apted, "almost to the point where you want to say: 'Don't tell us that, we don't need to know.' Others are buttoned up, but you always know it. It's not that people are lying to you, it's just that they're not coughing it up. That's frustrating, but you also know they're not getting away with it and they're not doing themselves any favours."

Seven Up! had a political agenda, but that soon gave way to an attempt faithfully to follow the rhythm of people's lives. "The overt politics evaporated," says Apted. "Society's changed. The politics of the films are now their lives." He differentiates his films from the spate of reality TV shows that have flooded British screens in the past decade. "I've always fought to distinguish between documentary films and reality TV. I had to explain when we did 49 Up that reality TV puts people in unusual or contrived situations and sees how they respond. What a documentary does is get a snapshot of what the reality is."

Apted, who is based in Los Angeles, has had a very successful career making feature films, but recognises that the Up series will be his epitaph. "This is the most original of the things I've done, and it's also frameworked my career. It was the first job I had and it's likely to be the last. It has been tremendously influential in my life and I feel it's unique. I don't think anyone will ever do this sort of thing again."

Jackie Bassett is one of three friends from the same east London school selected for Seven Up! Photograph: Murdo MacLeod

Jackie Bassett

Jackie Bassett was one of three friends from the same school in the East End chosen for Seven Up! Sue, Lynn and Jackie were inseparable then, and have kept in touch since. Jackie has had the toughest time of the three – divorce, single parenthood, the recent death of her partner, and rheumatoid arthritis, which means she can no longer work – but she remains buoyant, funny, feisty. She moved to Scotland 20 years ago and now lives in a flat in Motherwell. In 49 Up she rounded on Apted for being judgmental, suggesting he made too much of her illness and concentrated on what she couldn't do rather than what she could.

How truthful is she about her life? "There will always be parts you keep back," she says. "You have to do that." She never revealed the identity of the father of her first son, Charlie, born when she was in her 30s after her marriage had broken up. Nor did she explain why she separated from her later partner Ian, father of her other two sons. So how full a picture do we get? "About 90%," she says. "The only things I've kept back are the very private issues that would hurt people around me if I made them public. I know there are people in this programme who have made just about everything public" – she is thinking of taxi driver Tony Walker, who in 42 Up owned up to infidelities – "but I think they now regret those deeply. I don't know how you could do that on national television."

Why did she give Apted such a hard time in 49 Up? "When we were younger he would ask questions about our personal lives that he wouldn't dream of asking Andrew, John and Charles [the prep-school boys]. It was a class thing. But I don't think even he realised he'd done that. He had certain ideas about the way things should go, and for a long while he couldn't deviate from that. Last time we did the programme I was much more confident in myself and in my home situation, and I just decided that today's the day that I tell him exactly what I'm thinking. He was a bit shocked by it." She refuses to be seen as a victim. "I live with the rheumatoid arthritis, but it doesn't rule me. Michael believes that's the total reality of my life, but I don't want to be seen that way."

Is the Jesuit saying that underpinned the original programme correct? Could these lives have been predicted at seven? "I think it's probably true," she says. "I've turned out pretty much as expected." In 49 Up, Bassett asked Apted whether he thought she'd turned out badly. Did that really bother her? "It's not that I worry about it," she says. "I was pushing his perceptions again. I think he had an idea of where we would all end up. Sue [who has a job as a university administrator] has exceeded his expectations, and I don't think I've achieved his expectations. But I know I've had a good life."

Symon Basterfield is now a foster parent who has cared for more than 60 children. Photograph: Teri Pengilley

Symon Basterfield

Symon Basterfield's life has been a triumph against the odds. He never knew his father, his mother suffered from depression and he was in care when Seven Up! was made in 1964. His father was black, his mother white, and he suffered prejudice from both sides. His first marriage, which produced five children in less than 10 years, ended in divorce. His life was in such chaos with his divorce and the death of his mother that he chose not to appear in 35 Up. But he remarried, had another son and he and his wife became foster parents. More than 60 vulnerable children have been cared for at his neat house on the edge of Heathrow, where he works as a freight handler.

"At 14 and 21, I did think the programme was an intrusion," he says, "but afterwards it goes. It's like anything – we're today's big news when the programmes go out and then it's back to normal. It's not like we're constantly watched." The programme-makers keep in touch, but contact is distant because they want to be surprised when they return.

Why did he sit out 35 Up? "I was going through a part of my life where I realised that some of the things I was doing weren't right for me. Normally I would just go with the flow, but I felt I'm not me at this stage. I couldn't have been honest. It's a period I don't want to talk about." He is still unwilling to talk about his divorce or say anything that would hurt his first wife.

Is it a weakness of the programme that when life becomes really difficult, the subject pulls out? "No," he says. "If you watch Big Brother and the jungle, it becomes cheap. You might be watching Jeremy Kyle. People think they'll get their case over, but they don't because their case might be one, two, three, five, 10 years long, and you're trying to pack 10 years into 15, 20 minutes. Some things are better left unsaid. If everything was shown, it would be detrimental. We would drown people who were watching. They'd be soaked up in it at first, but after a while it would press in on them. It would not leave enough room for them to think about their own lives."

He says that what began as a sociological exercise became a more human engagement with individual lives. "In the beginning you could see exactly what they wanted. I noticed that with Paul [who was in the same care home] and I. We were supposed to have aspirations to what we wanted in life, but the boys from wealthy backgrounds were encouraged to say their lives were plotted and planned. It was all hopes and dreams for us, but their lives were mapped out."

And did they prove the premise – that you can take the boy of seven and see the man? "In some ways they did," he says, "but also no, because you're not just dealing with a sociological experiment. It's real life as well, and those people have developed for themselves. Nothing is clear-cut." Symon is living proof of that. The shy, disadvantaged boy of seven who became a bit of a star.

Bruce Balden has taught in the East End, Bangladesh and now at an independent school in St Albans. Photograph: Graeme Robertson

Bruce Balden

Bruce Balden, who teaches maths at St Albans school, was at pre-prep school in 1964 when he was selected for Seven Up! If he had been given the choice, would he have taken part? "I wouldn't have minded," he says. "They haven't portrayed me in an accusatory way. It might be different for some of the others, who've had a rougher time than I have. Cardinal Richelieu had that famous saying: 'Give me six lines by the most honest man, and I'll find something in there to hang him.' If they film you for a couple of days, they can portray you any way they like, but they've always portrayed me quite nicely."

Does he tell the truth? "There are one or two things I covered up, mainly because they might have hurt people close to me. But on the other hand I have said one or two things which have been revelatory, and I've regretted saying them. I've made comments on family members which I regretted, but they're good at spotting if it's too raw.They're not sensationalists, which makes it easier to come back to."

Balden's father was in the RAF and then worked in Rhodesia. His parents separated, and he came back to the UK with his mother and boarded at a school in Hampshire, living with his aunt in the holidays because his mother worked.

He seemed a little lost in Seven Up! – "If I watch myself at seven," he says, "I don't think that's me" – and only when he married and had two sons did his life take on a more settled aspect, after two decades in which he had appeared to be attempting singlehandedly to put the world to rights, teaching at a challenging school in the East End and doing a stint in Bangladesh.

How did his mother react to the programme? "The last time, they were filming me outside my old boarding school. I said I'd boarded here because of family circumstances, and I know my mother felt a bit guilty about that. I didn't have time to say there were advantages and it was a difficult situation, and it came across a bit glib, in effect blaming my mother when I wasn't really." His father saw the programme when he retired to the UK, and Balden says he was proud of it. His sons love it, his wife accepts it – if she objected, he says he would pull out – and the boys he teaches only rib him gently.

Does he feel any guilt about leaving the East End for his present leafy surroundings? "Not really," he says. "It was life circumstances, and what was best for me and my family. You have to live your own life." Even when millions of others are living it alongside you.