Iris, a documentary by legendary filmmaker Albert Maysles, is the portrait of a woman who came late to the public eye. Iris Apfel is 93, and she's never been busier or more in demand.

She created her own niche without ever intending to. When she was in her eighties, the world discovered her distinctive style and philosophy, the way she mixes colours and textures, designer and op shop items, eras and layers. This eclectic approach seems perfectly natural to her. "Why not?" she says, on the phone from New York. "That's life."

Apfel's style – expressed through her clothing collection and the way she wore it – was the subject of a museum retrospective 10 years ago at the Costume Institute at the Metropolitan Museum of Art. Since then, she's been muse, fashion inspiration and media star, and has had a five-year association with a university fashion course. Now she has several design projects in train, and a book on its way.

Apfel had a long career in design before her emergence as a public figure with a beguiling sense of style and play. She and her husband, Carl (a warm, appreciative presence in the film) founded Old World Weavers, a luxury textile business that specialised in restorations and had several White House commissions.

It took a while for her to agree to be in Maysles' film. "I knew him by reputation, although I didn't know everything I should have, I'm not the biggest film fan." she says. "But he called me one day and said he would like to do a documentary about me." She told him no. "I really don't need it, nobody knows who I am, I don't have anything to sell, and I don't have any ego problems. And I bade him a fond farewell."

He called back, and she said the same thing. Then she told a fashion friend about the call, "and she really laid me out. She said, 'what the devil are you doing, he's such an important man. People would drop dead if he asked them to take a still image, and you're denying him a documentary. Shame on you'." Over four years, on and off, they got together, and he and his team would film her. Maysles, one of cinema's most celebrated documentary filmmakers, died in March 2015.

The Apfel look has some recurring elements (oversized round glasses and armfuls of bangles) but there's nothing predictable about what she wears or how she puts things together. In the documentary, we see her in one extraordinary outfit after another, but that's not the way she looks every day, she says. "I just shrivel when people think that all I do is think about getting dressed." And the last thing she would advocate is being in thrall to fashion. "It's better to be happy than well-dressed," Apfel says in the film, and it's something she firmly believes. "People obsess about fashion when they shouldn't."

She remains a busy, driven figure. She's an assiduous user of her mobile phone, but steers clear of every other device and all forms of social media. "Technologically I live in the 17th century." The phone is always ringing, and being in demand has its pressures, she says. "It's tiring and draining. I'm thrilled to pieces and grateful that at my stage of the game people are being so generous and kind with me, and I can't tell you I don't like it, because I would be a liar." She's learning to say no, even to things she would love to do, because of the constraints of time.

"When I was very young, I experimented a lot, and I learned. But it takes time, you have to find out who you are and what you're comfortable with, all that kind of stuff. And a lot of people don't realise that, they want instant everything. They want me to tell them how they can have style. In three easy sentences." She laughs. Yet in a way, she can boil it all down. "Attitude. Attitude. Attitude." That's what it's about, she says. Just as "fashion is not a runway bubble, it's a mirror of society", your style is an expression of who you are. You can't buy it and you can't acquire it from someone else.

Iris is in cinemas from August 13