A couple of days after I talk to Daphne Selfe, Paul the photographer sends me the pictures he had taken on the day. I have a look and immediately email him back. "She's doing the splits?" I say. I'm not sure if it's a question or an expression of astonishment.

"I know," he replies. "Incredible!"

Daphne Selfe is a model. She is tall, elegant, very English in a jolly-hockey-stick, Henley-Regatta-and-Ascot kind of way. Oh, and she's 87 years old.

Yes, 87, and still doing the splits; 87 and still busy. "I'm still working really well," she says. "Busier now than ever really."

She's already been to Australia on assignment this year to model spectacles and to wear fabulous clothes. "Dior and Chanel," she says approvingly. She's set up an online modelling academy and even found time to write her memoir. The Way We Wore, it's called. Not that her wearing is in the past tense.

Daphne, I say when we start talking, the question is why? Why are you still modelling? Why are you getting up at five in the morning to catch a flight? Why aren't you sitting down with your feet up watching endless episodes of Murder She Wrote?

"Why would I want to put my feet up?" she says. Maybe I'm imagining the incredulity in her voice. Maybe not.

"What a waste of a life," she continues. "I don't want to waste a minute of this life. It's much too interesting. Modelling takes me all over the place. I've never travelled so much."

She is at home in Baldock in Hertfordshire today. Apart from doing the splits she's spending the day cooking and talking to me. Mostly about modelling then and modelling now. I think she might enjoy it more today. It certainly sounds a little more glamorous. Back then – then being the start of the 1950s – it was just a bit of fun, something to do before life got serious.

For five years at the start of that decade she wore dresses for a living. She stopped when she married and had a family. After a few years of motherhood she started another life as a TV and film extra. There was still the odd modelling job. Advertising Corn Flakes or Berkertex and Dannimac. But Vogue wasn't calling.

But then in 1998 she was hired to work the catwalk for Wayne Hemingway's Red or Dead label. She was 70 years old at the time. And suddenly things kicked off. There were shoots with Mario Testino, David Bailey, Rankin, Mary McCartney and Nick Knight wearing Hussein Chalayan (and no underwear). She's turned up in Marie Claire and, yes, Vogue. She even dressed up as Madonna (wearing Jean Paul Gaultier's infamous bustier, no less). All a far cry from appearing in the Reading and Berkshire Review (her first cover).

In many ways Selfe was at the forefront of fashion's interest in older women. That interest is, if anything, more obvious 17 years on from that Red or Dead show.

Selfe is not alone. Fellow 80something model Carmen Dell'Orefice is a regular in the fashion pages. And these days Joni Mitchell (71) is being photographed by designer Heidi Slimane to promote Saint Laurent. And Chanel are using the 80-year-old writer Joan Didion as the face of its new ads.

When it comes to fashion, old age is having a moment. Actually, decade might be more accurate. Writing about this very subject in the Spring/Summer issue of monster fashion magazine Pop, the magazine's co-founder Ashley Heath quoted fashion PR Emily King on that Didion ad.

"This isn't the first time that fashion has flirted with age," King wrote, "but it does feel different doesn't it? There is no hint of a specific 'senior style' – no overwrought eccentricities, no purple, or fringes, or weird hats. Even better there's no sexy-in-spite-of, not a sniff of GMILF. It's just a photograph of a brilliant 80-year-old writer looking stylish, interesting and a shade intimidating."

What's happening here? Simple really. The fashion industry is recognising that the baby boomers may be retired but they are by means retiring. They also have a lot of disposable income.

"Everybody's getting older," Selfe points out. "The population's getting older. They're older, they're fitter than they used to be at that age. They retire earlier, perhaps. And the people who've retired want clothes. And they've got more money."

And that's why Chanel is using Didion. That's why Selfe is still working. The grey pound. She is a representation of the economic muscle of older people. She's also a representative of their aspiration to live a more stylish old age.

And so Selfe finds herself on the books of an agency who also represent models 50 and 60, even 70 years younger than she is.

What's her relationship with the younger models? "They're no threat to me and I'm no threat to them. So we get on very well. And they all say, 'we want to be like you when we get older.' And now I find I can do things like help them occasionally. I'm aspiring to be an inspiration."

Daphne Selfe was born in 1928 into a world of respectable gentility. Her father was a teacher, wealthy enough to be able to run a house employing several servants. Life was all tennis games and dinner parties. Her parents lived near Sir Ernest Shackleton and the portrait artist Gerard Leigh Hunt (who painted Selfe's mum when she was 19; a portrait that now hangs over Selfe's bed).

This all came to an end in 1931 when they lost their money after Britain came off the Gold Standard. But clearly they weren't reduced to grinding poverty. Selfe's dad was, she says, a very capable teacher and was always employed. Certainly, a few years later, her parents were well enough off to send their daughter to boarding school.

The way she describes it, Selfe's school years sound like a mixture of Enid Blyton and Norman Thelwell cartoons. The war apart, it was all comfy school knickers (she'd wear them now if she still had them, she says. "I'm quite thin and I'm always cold, so I never wear what I call scrappy underwear"), lacrosse, horse riding and, later, hunt balls.

She was a bit obsessed with horses, she admits. Wanted to make a career out of that obsession. Only problem was, they kept kicking her. Sometimes in the head. So instead she got a job in a department store in Reading.

Modelling was never a goal. She just fell into it. "The job with my horses hadn't worked out well. I came back home, worked in a store, did a competition for the local magazine and I won. And that sort of started me off. Then in the store the girls came down to do a fashion show from London. They were one short, so I filled in the gap and they said, 'why don't you come and train to be a model?' So I did."

What did your mother think of that, Daphne? "She thought it was a lot better than being kicked about by horses. Which is a fact, isn't it? Although it's still dangerous, but in a different way."

Ah yes. In The Way We Wore she does write at one point: "I've met a lot of wolves in my time and I can't stand them."

"I've escaped all that, thank you very much," she says when I bring it up. How often were you bothered, Daphne? "Occasionally. They want you as their girlfriend or to take you away for the weekend. I was pretty cool and rather forbidding when I was young, I think."

Modelling in the fifties would have been unrecognisable to the Kate Mosses and Gigi Hadids of this world. Most of Selfe's work was as a house model, modelling clothes for dress manufacturers in front of potential buyers. "You were a model. You're not a person at all," Selfe recalls. "You're just somebody with clothes on, showing customers. It's a very impersonal thing really."

There was no real training. She did ballet, had good posture and looked striking (though she worried about her broad shoulders). That was enough. Maybe it was a less body-conscious era, she thinks. She was even an artist's model for the sculptor Barbara Hepworth for a while. "That didn't bother me. You're a piece of furniture, that's all.

Actually, she says, posing for Hepworth was fun. "I wanted some money for a holiday and I phone her up and said 'do you want a model?' She liked having dancers as models so that was good for me. I went and sat for her for a few sittings."

It was a less cosseted age. Models did their own make up back then, brought their own accessories, jewellery, hats and gloves to jobs. "I couldn't do it now. Well, I could with a wheelie."

The idea of a model as a personality was unknown back then. There was the odd society model like Barbara Goalen, but even she only turned up in the pages of Vogue and Harpers Bazaar. There were no 3am gals, following their every move. "It was only in the sixties they began to get known," Selfe points out.

How was modelling perceived then? "It was considered alright. It wasn't a proper job. You just did it for a bit and then got married."

She did that in 1954. To Jim Smith. They were together until his death in 1997.

After they married she devoted herself to her family. Modelling was put aside. And by the sixties she began to think her moment had passed. "I wasn't the right sort. I wasn't like Twiggy … shall we say? I just wasn't that type."

And then 30 years later – years filled with background appearances on TV (her husband worked for ATV) and in films, things took off again.

Modelling is such a different world now, I suggest. "Many more people and much younger people," she agrees. "Much younger, much thinner, more jobs. They travel every year. They never stop. They're all over the place."

The pressures are much greater presumably. "Yes. There's much more pressure because they have to be so thin. Because designers just want thin models. Clothes do look better on a slimmer person. But if you've got a good figure and you stand up straight you're going to look good. Posture is everything. You really don't have to be a size six. I'm a size … What am I? I'm only a size 10."

Fashion is obsessed with size zero. "I know. I know. My bones are bigger. Even as a skeleton I wouldn't pass that one."

She laughs at the very idea. She laughs a lot. She laughs when I bring up Madonna's bustier. Daphne, when they asked you to wear the Jean Paul Gaultier bustier for charity what was your original reaction? "'They don't want me for that. I'm too old.' But, no, they did. I'm always game for a dress up and fun. I'll do anything, especially if I'm getting paid. Oh, but this was a charity thing. I didn't get paid for that. But what the hell."

Is fashion too hung up about sex? "Oh dear, yes. I think so. It wasn't like that when I was first modelling in the fifties. I suppose it is a bit now. I don't know much about that bit. I'm past that. That's actually the nice thing about getting old. Because there's no pressure in that direction. You just have fun."

She does sound as if she is having a lot of fun. She loves clothes, is a terrible hoarder and loves the chance to dress up in labels. Does she have a favourite? "I like Etro because he's a bit wacky - is it he or she? – they're a bit unusual. I do like unusual things.

"I was looking at Victoria Beckham's collection and I thought, 'I could wear some of those'. I couldn't afford them but I like what she's doing."

She used to make her own clothes, because she didn't want something that was off the peg. But not so much now. She doesn't see as well as she used to (a spot of glaucoma). Still, she keeps fit, walks a lot, cooks, keeps busy and every so often someone will hire her to fly to China to model.

Is the 87-year-old model still the same girl she was when she started modelling in 1950? "Yes, I think I am. I was always game for anything. Earn a bit of money if I can. Meet people. Travel. I'm a curious person."

What has life taught you, Daphne? "Make the most of it and live in the present. You never know what's around the corner. Make the best of it. It's not very long. Be curious. Be positive. Don't moan. Keep healthy by eating the right things."

I'm failing on all counts I tell her. Buck up, she says, you've got to enjoy life. She is clearly enjoying hers.

"I do. Well, it's long is it? I've got to the end of it almost. I might have … What? I dunno. Another five, ten years perhaps. It soon goes."

Indeed it does. We all go out of fashion eventually.

The Way We Wore, by Daphne Selfe is published by MacMillan, priced £16.99.