You might think, from looking at the ever-youthful Lee Janogly, that she has a Dorian Gray-like portrait in her attic, doing the ageing for her. Instead, her attic is a large exercise studio, which goes some of the way to explain her incredible energy and sprightly appearance.

It is hard to believe she is 80, as she demonstrates a variety of moves that involve twisting her body like a pretzel, and stretching her legs in a way only a professional dancer or a gynaecologist would be accustomed to seeing. If this is old age, then the rest of us really have nothing to fear.

That’s the theme of Janogly’s latest book, Getting Old: Deal with it. In a good-humoured, witty and sometimes moving rage against the dying of the light, she offers diet and exercise tips on how to stay healthy for as long as possible.

“I wanted to dispel the notion that older people are slow, frail, dim-witted and a burden on society,” she says. “Admittedly, some of them probably are, but most of them aren’t. The old people I know are very busy all the time. They are either looking after grandchildren in the school holidays, going to the gym, doing charity work or they are playing bridge with friends. We are not sitting around knitting covers for our hot water bottles. We are not the frail pensioners the media makes us out to be.”

She says that exercise is a key way to stay healthy way past the time you receive your bus pass. “If you don’t exercise and don’t remain supple then you do look older. You don’t have to go to the gym — I know it looks like a torture chamber, but there are classes for older people. You are never too old to start. Each time you go, you will see the same people and get to chat. It’s half physical and half social, which is so important. They say loneliness is the worst thing for older people — that causes depression. Just go along and have a laugh. Even if you can’t do it, it doesn’t matter; you’re not auditioning for anything.”

Her own journey to becoming a personal trainer and diet counsellor began when her mother suggested going to the local keep-fit class to lose some post-pregnancy weight. Having had two boys and three girls in the space of seven years, she was keen to get back into shape. She started going to the Dance Centre in Covent Garden and one day offered to lead a class when the teacher failed to arrive.

Her interest in jazz dance and exercise came at a time when there was a huge resurgence in all forms of dance, thanks, in part to the film Saturday Night Fever. Thousands took to leg-warmers and lycra in a big way, and at one point, she was teaching between 80 and 90 people in one class. She opened her own studio in Golders Green in 1982, calling it, appropriately, All 
That Jazz.

She is refreshingly frank about the effects of ageing upon women’s bodies (everything either sags, wrinkles or dries up), acknowledging she resorted to plastic surgery 30 years ago. She also underwent a hysterectomy, when, as she puts it, “my womb was practically dropping out on the floor”.

“Yes, I had a face lift when I was 50, but that was because I felt my face was dropping off and gathering under my chin. That, and a hysterectomy, were honestly the best things I’ve done under anaesthetic! I don’t think I look any different from anyone else my age— I’ve got the requisite number of wrinkles, and as far as looking young, I think that this is what 80 looks like.

“I try to be as honest as possible, because ageing is what happens and it is normal. I try to put a positive spin on it because eventually we are all going to develop some horrible condition from which we will die. People who research these things, like Age UK, found that most people stay reasonably healthy up to the age of 65, and after that there is obviously more chance of getting something like heart disease, dementia, stroke or cancer.

“It is not very pleasant living for a long time with a debilitating illness, so now the health professionals are concentrating on what they call ‘wellness span’, which is trying to find ways for older people to live as healthily as possible, for as long as possible. When they eventually get ill, whether it is mental or physical, that will happen much later in life and be mercifully short.”

It is something she had to deal with at first hand five years ago, when her husband Maurice died of stomach cancer. She writes about her devastation in the most moving chapter in the book.

“I actually handled his death very badly. Because I was trying to be cheerful for him, when he was in hospital I said, ‘I’ve got five children, I’ll be fine’ so when he finally died, I fell apart. Obviously I’d never experienced grief to that extent, but I just could not stop crying. So I hope that chapter will reassure other women going through the same thing that it is normal. They are not going mad because you think, ‘What’s happening to me, I can’t function.’ But it’s okay, and it really does pass. For me, it took two and a half years before I could wake up and think ‘I haven’t cried today’.”

She says that everyday things that people take for granted — like someone on hand to zip up a dress — can be particularly difficult for a widow. “Like going out for dinner with friends. You go out in couples, and suddenly there’s two other couples and there’s you. Someone said to me, keep some £10 notes in your bag and when it comes to it, put the requisite number on the table and say, ‘Look, I want to.’”

Those who are left behind inevitably face their own mortality and she advocates putting in place a Lasting Power of Attorney for Health and Welfare, so that relatives know your exact wishes should you become mentally or physically incapacitated.

“You can set out specific instructions about your care, in case you get dementia or a stroke, and you can’t make those decisions for yourself. If that happens to me, and I get cancer, I wouldn’t want chemotherapy, for example. Why would I want to prolong that? It is in doctors’ nature to cure, that’s what they’ve been trained to do. So they often don’t take into account whether their treatment is the best thing for the patient.”

You can see her own instructions to her children in the book, and it does make for rather sombre reading, but she says she is an optimist at heart.

“I’m not going past 90, but I do want to live to the full until then. I would just like to say, ‘OK, that was very nice. I’ve had a lovely life, I’ve been blessed with healthy children and grandchildren but I’m ready to go and join Maurice now.’”

Getting Old: Deal with it is published by Mensch Publishing, price £9.99