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Agnès mentions all this within a couple of minutes of settling down in the chintzy salon of a hotel overlooking Kensington Gardens. Frank, friendly and refreshingly unfashiony, she is gobbling down white-bread finger sandwiches with a glass of red wine. "I've been at Frieze [the contemporary art fair] all day, and haven't eaten," she explains.

At 72, she still looks like a naughty cherub, with golden ringlets and a mischievous smile, and she is wearing a black shift dress and black cashmere jacket from, of course, her own label. "I never go shopping. I have no time for that. I work for eight, 10 hours a day. I go straight to my studio when I wake up. But I like to be out in the evening, too, with my friends. To listen to music, or go to see art. I'm very active; I'm never bored. I'm a very greedy person, as you can see," she says, waving her hand towards the sandwiches. "I am greedy for life. You only have one life, so you should make the most of it."

This appears to have been her motto from an early age. Born into a respectable middle-class family from Versailles – her father was a lawyer with a passion for art – the young Agnès lived up to her maiden name of Troublé.

At 17, she ran off with her student boyfriend, Christian Bourgeois, married him and quickly became pregnant – with twin boys. "I had twins for my 19th birthday! It was very animal. I was breastfeeding one, holding the other."

She divorced the following year – "I married too young to stay pure" – and moved to Paris. Her parents' disapproval meant she felt she couldn't ask for any money, so life was a struggle. She sold her wedding and engagement rings and ripped up her wedding dress – her very first design – to make curtains for the twins' beds.

Then a fashion editor at Elle magazine happened to spot her at a flea market, liked the clothes she was wearing, and asked if she'd like to become one of their junior stylists. She stayed for a couple of years before realising that she would rather design clothes than style them, and left to work as a freelance designer.

In the early Seventies, Agnès started her own label. The clothes were the antithesis of mid-Seventies fashion, which was dominated by bad taste and polyester. Instead, she took plain white garments that she then dyed and sold straight from the clothes line. Her first shop, which opened in 1975 in a former butcher's in Les Halles, sounded a riot. Customers were allowed to scribble on the walls, and at one point there were 35 birds flying freely around (she had left open the cage for her children's pet birds, and they had subsequently nested).

The "b", borrowed from her first husband's surname and always lower case, "suited me", although by then she was on to her second. In fact, there have been three husbands in all; five children; and, to date, 16 grandchildren. "I was a grandmother when I was pregnant with my last child," she says matter-of-factly.

Still, Agnès's complicated family life – she remains good friends with her former husbands – has never got in the way of her business. Her cool, uncomplicated aesthetic became a word-of-mouth success. (She has always refused to advertise: "I'm of the '68 generation," she says, referring to the student protests of 1968. "I think advertising manipulates people.") More stores opened up in France, New York, Japan and, in 1987, London.

There are now 300 Agnès b boutiques around the world, with more opening in Asia. Her clothes tend to suit those with a slender frame. "That's why we do well in Japan and China, Hong Kong and Taiwan. I did a trip to Taiwan last year; they were crazy about the fact that I was there."

Keeping her designs simple, she thinks, has been key; clothes that you wear, not ones that wear you. "That's why I don't like fashion so much. I love clothes, and I love designing clothes. I love looks that aren't dated. My customers are from so many different ages."

She also dresses several famous people. David Bowie, for instance, whom she saw in concert in Paris, and was horrified by his outfit. "It was about 20 years ago. He was wearing a brown suit with pleats, baggy. He looked like a German, not rock'n'roll. I sent him a pair of black leather jeans with a note in the pocket saying: 'You should stick to black and white.' He ordered four more pairs. And then I dressed him for 10 years on the stage.

"I dress a lot of people," she continues. "David Lynch [the film director], I have dressed him for a long time. He sometimes takes some pieces of mine for his films."

The same goes for Quentin Tarentino, who used her clothes in Reservoir Dogs and Pulp Fiction (remember Uma Thurman in a white shirt with oversized cuffs and ankle-skimming black trousers, dancing with John Travolta? All Agnès).

"Quentin Tarentino liked my clothes because they're not too fashion, too designed. Harvey Keitel still has my Reservoir Dogs suit, even though it is full of bullet holes! And Travolta asked me for the same jacket 10 years later – black linen with a leather collar – because he loved it so much he wanted a new one."

When it came to directing her own movie, she didn't ask any of her friends in film for advice, possibly because she was a little nervous about making it. She'd had the idea for a while. "I wrote it a long time ago, very quickly, in two days."

Despite already having some experience in film, both shooting short films of models for her collections, as well as co-producing movies with directors such as Harmony Korine, she didn't have the confidence to make a full-length feature of her own. "I didn't know if I dared to do it, and I didn't know if I had the time. But my friends and colleagues encouraged me. I wanted to express myself in a way other than fashion. I've always been interested in art and in cinema. I've had an art gallery since 1983."

^{Agnes B: 'It is my duty to do the best for my country' (Geoff Pugh/The Telegraph)}

Indeed, alongside film, art is her other great love. She owns more than 3,000 works including pieces by Henri Cartier-Bresson and Keith Haring. Does she have a favourite? "I love them all. Many are from people I've discovered early on, such as Jean-Michel Basquiat, so I have pieces from the beginning. All my friends are artists. I always say I don't know many rich people."

This might be why she tries to be such a reasonable boss – her staff work 35-hour weeks and have six weeks' holiday. "Some women don't want to work on Wednesdays to be with their children, for instance, so they only work four days if they want. I have five children so I know what it's like."

She also makes many of her clothes in France. "As much as I can, around 40 per cent. We have so many unemployed people, so I feel it is my duty to help, as a citizen, to do the best for my country."

She remains politically engaged. "I always read Le Monde – I've been reading it since I was 17. I'm sad about the current state of France, though. It's crazy because we have just won the Nobel Prize for economics. We should be listening to that guy!"

Yet Paris is still her home. Right now, her youngest child and 16th grandchild are living with her. "He's lovely," she says of her grandson. "Blond hair and blue eyes. And he's very positive. His character is very much like me and like my father. It's lovely having a child in the house, having new babies around me."

There are no plans to retire. "I love history and I love memory but I have no nostalgia. The future is something you can always do something about. You can do nothing about the past."

You can commemorate it, though. Next year marks 40 years since she opened her first shop, and she is planning a book ("the story of Agnès b"), as well as a series of exhibitions. Another film is also in the pipeline.

This youthful drive, she believes, is what keeps her looking, and acting, so remarkable for 72. "I'm not twisted – I'm very straight-forward in my relationships. I spend so much time with young people. I love to swim; I love red wine; I love to cook. I'm very lucky with my life. But you make your own life, too, because sometimes it's hard. It's how you deal with it that matters.

"But you know," she adds with a twinkle in her eye, "I'm still a rebel."

'My Name is Hmmm…' is in cinemas now