Joan Bakewell's home is not like any old person's house I've ever visited. Although she has lived on this North London square for 40 years, through child-raising and two long marriages, it bears none of the dotty accretions, the clutter of prolonged habitation. Its decor, the magnificent kitchen, all the gizmos, are new, even state of the art. Of that which is old, she seems to have kept only what is most beautiful.

But there aren't many old people like Dame Joan. It is a shock to think she is 75. But then, as the Government's new Voice of the Old she aims to make us wonder anew what 75 means. And such a poster old girl she is! Not because of what she'd term "giddy" stuff like her enduring elegance, but her absolute refusal to be disappeared. Besides chairing this and that, a forthcoming Radio 4 series on ethics, endless articles for publications including The Times, Bakewell has just written her first novel. True to reputation, All the Nice Girls mixes history - the "adoption" of a ship by a girls' school during the Battle of the Atlantic - with some juicy sexual transgression.

Since her second marriage to the theatre producer Jack Emery, 12 years her junior, ended in 2001 she has lived in the big house alone. To sweep away lingering divorce melancholy she had the place refitted into a ladylike bachelor pad.

The greatest fear of age is being alone, but this too she seeks to reassess. "I think now I would find it hard to live with someone," she says, "to take on the responsiblity, that degree of caring for them, to make sure their lives ran smoothly and they were happy. I did all that and it went wrong and I don't want to hazard that much again. The pain of it was too much." Now she relishes the end of marital compromise: the dreary haggling about holidays or paint charts. "The downside is missing the one for whom you are the unique person in the world. That's really the essence of marriage: the other who regards you as their pole star. But then," she reflects, "there are other ways in which you can show your love."

Her two children and six grandchildren are close by, but she doesn't just mean them. "I've also learnt to love myself. I like my own company now, when I used to be lonely alone. The sense you are unique and self-sufficient is very rewarding." For all her achievements, her first-class mind, her groundbreaking role as one of TV's first women presenters on Late Night Line-Up- interrogating Vaclav Havel or Allen Ginsberg in skirts so short no serious presenter could get away with them today - what will guarantee her immortality is her seven-year affair with Harold Pinter, thinly disguised in his play Betrayal. There is a photograph taken while she interviewed him. He - saturnine, intense - leans foward, while her body language opens to receive him. Both are smoking. As Bakewell's agent remarked, it is a very sexy picture. How well did they know each other then? "Pretty well," she says and chuckles dirtily. I might have guessed: it was at the height of their affair.

Nothing in her autobiography, The Centre of the Bed, is so vivid as the passages about Pinter. Her memories vibrate on the page: the green velvet frock that she wore when they first met, the flaking paint on the park bench when they first touched, a snatched day in Paris. She recalls Pinter's wife Vivien Merchant, her "self-contained and proprietorial pride" with a snippy mistress's envy: "I didn't know then she was an actress but I might have guessed."

Bakewell makes the Sixties sound a golden age for adultery: you could motor across town with no traffic or parking rules, enjoy a horny half-hour, then be back to pick up a child from nursery, with no redial button or mobile phone records to catch you out. When Pinter, years later, sent her the script of Betrayal she describes "an appalled sense my life was being raked over". But I detect a satisfaction in a woman who so worships words and ideas, to have generated art with her body, her charm and allure. And now Pinter is dead and while Bakewell has declined all offers to write about him, it is clear he still burns in her heart. She says: "Even within recent years, he'd sometimes say, 'We had a good time, didn't we?'" She adds, emphatically, as if he's in the room right now. "'Yes we certainly did!' And it was very nice, and then we'd smile."

They continued to meet over the years. "We were distant, married to other people, in different worlds, but we just kept an eye on what each other was up to." They had lunch about two months before he died and spoke on the phone a few weeks later. Her eyes are filling when she says, "I don't want to talk any more about it, except to say it does linger, it was very good and I was shocked by his death even though I knew he was dying." She was not at his funeral. "I felt it was for Antonia [Fraser, Pinter's second wife] and family and I was outside that. And, anyway, my own grief is private to me."

Why the affair still intrigues, besides the fame of its participants and the brilliance of Betrayal, is how it embodies a period when all social conventions were challenged, including marriage itself. When Joan learnt that her husband, Michael Bakewell, was conducting an affair, she confessed to her relationship with Pinter. For the sake of their two children and because they loved each other, they agreed to keep their marriage if not exactly open certainly ajar.

They had married young, at 22, and as Joan points out was then the only way to have a non-furtive sex life. "Michael and I were living in a climate of trying out different ways of being married. We thought it extraordinary that you're meant to find only one person in the world attractive."

Yet why did she never tell Pinter that Michael knew? Did she fear pricking the bubble of their secret? "It might have taken our lives off in a different direction. God knows what direction." In the end, it was Bakewell's admission that ended the affair. The Betrayal of the title is not adultery, but the dishonesty between the two men. Michael and Harold were friends (but not as close as the men in the play): Pinter was incandescent at being deceived.

Joan Bakewell is an odd participant in a bohemian love triangle. Her family was solid, aspiring, lower middle class. Bakewell went to Stockport High School for Girls, which like the school in her novel adopted a merchant ship, the pupils writing to sailors. She was a "nice girl" too: school swot, head girl, went to Newnham College, Cambridge, her northern vowels sculpted by elocution lessons into precise BBC diction.

But her home life was poisoned by her mother, a brilliant woman who left school at 13, never achieving her ambition to be an engineer, a male-only profession. Bakewell attributes her mother's depression, petty snobbery, vicious outbursts and hypercritical nature to frustration at being confined to the home. Dead fifty years, Bakewell says the only approval she still craves is hers.

She wanted to escape her mother, never grow to be like her and wanted what she knew didn't exist in Stockport: a life of the mind. And of the body too. She found it all in Sixties London and you sense a hunger in Bakewell: nothing would stop her getting what she wanted, whether TV job or man. "I always say that will matters as much as talent," she says. "And I do have a strong will. And if I want it, I will get it and if I don't get it, I think, well, I didn't want it enough."

Even at 75, she still has drive, energy and an exhilarating appetite for the world. One of her favourite pleasures is attending the Times news conference just for the whirr and debate. She loves fashion - today she is wearing a knitted cotton jacket with big shell buttons and foxy high-heeled boots - and observing how young people on the street put together new looks. Her son is always telling her off for swearing in front of her grandchildren. Writing of being a young mother she railed that, being stuck indoors, her brain was turning to porridge. No doubt she fears the same now.

I suspect that she'd love to be a peer, but she says, unconvincingly, that she hasn't thought about it. Her tip for old age is always having slightly too much to do. "And you must do Pilates," she says. Age doesn't frighten her so much as illness: the slightest symptom has her flying down to Harley Street. But she is working on the assumption that she will live to 95, eventually selling her beloved house to pay for care.

Can she imagine a last romance? "There are couple of friends I'm close to and if either of them were to move to centre stage - or centre bed technically - I would lose the relation with the other one." So Joan, two potential suitors, but you can't give up either? No change there then ...

Later, discussing Bakewell's role as Voice of the Old, she tells me of an idea to put a clipboard at the end of each care-home bed, bearing the CV of the patient: whether she was a teacher, or he was bank manager, the music they liked, how many children they had. That way the low-paid, put-upon care worker could get the satisfaction of connecting with the person, the humanity inside the now decrepit shell.

I suggest that they add a photo to the clipboard, taken in the person's pomp. And I think again of that picture of Bakewell and Pinter: in this youth-crazed world it helps to be reminded that older people were once libidinous, beautiful and hot.

[ Janice Turner joined The Times in 2003 from The Guardian, and writes
mainly, but not exclusively, on family matters and women's
issues. Her column appears on Saturdays ]