Annabel Ferriman discovers half a dozen factors shared by those fortunate beings who look much younger than their chronological age
When Antonia Barber made the mistake of confessing her age at a dinner party, her host grabbed her, pulled her over to a lamp and studied her face under the light. "No," he said. "I don't believe you. You can't be that old."

That was 16 years ago and since then, she has seldom mentioned her age. "If you are not careful, it can become the most interesting thing about you," she says.

Antonia is one of the super-young - those lucky people who look much younger than their years, in her case about 12 years younger. "At 62, I look and feel 50, so I think of myself as having a 20 per cent discount on age. After all, how you think of yourself influences how you are."

Mrs Barber adds thoughtfully: "Why should people born in the same year all be thought of as the same age?"

She and 3,000 others, between the ages of 19 and 102, have been recruited to a project on the super-young, set up by Dr David Weeks, a clinical neuropsychologist at the Royal Edinburgh Hospital. The project will last 10 years, during which time he will be looking at the ageing process and the connection between ageing and ill-health. It is particularly topical in light of the birthday last month of a 120-year-old woman in Arles, southern France, who still has all her faculties intact.

So what are the secrets of eternal youth? Why does Antonia Barber look so much younger than, say, the late Nicholas Fairbairn, who died at 61. In analysing the results so far, Dr Weeks has discovered five or six factors that these people tend to have in common.

The super-young are inclined to take regular exercise (and usually have done so since their late teens or early twenties), have happy marriages or partnerships, are often married to, or live with, someone younger than they are, and mix with younger people, either socially or at work.

Few of them smoke and, in the case of post-menopausal women, many take hormone replacement therapy. Their diets did not turn out to be particularly experimental but there were 5 to 10 per cent more vegetarians than predicted. The list will undoubtedly disappoint some people. As Oscar Wilde put it: "To win back my youth ... there is nothing I wouldn't do - except take exercise, get up early, or be a useful member of the community."

Dr Weeks explains how the project started. "I became interested in how some people age faster than others when I worked part time at a general practice in Edinburgh. I realised then how people's appearances were a poor guide to their chronological age.

"Then, in the late Eighties, a number of doctors produced papers, remarking on how young and vigrorous some people were late in life and how frail were some of the others.

"Some people with senile or pre-senile dementia look much older than their years, which raises the question as to whether Alzheimer's disease is perhaps more than just a premature cognitive ageing of the brain. Could it perhaps involve bodily ageing as well?"

He has recruited people through the media, asking anyone interested to send in a photograph. An independent panel assesses the age of the people in the photographs and, if a person's real age is considerably more than that estimated, the volunteer is enlisted. Dr Weeks's subjects, who, on average, look 12-14 years younger than their real age, then fill out a detailed questionnaire.

"We have observed how enthusiastic many women in their forties and fifties are about hormone replacement therapy. They are almost unanimous in singing its praises.

"Our data contradicts completely Germaine Greer's views and her condemnation of HRT. Although some women are happy to accept the natural ageing process, there are many now who want to put it off for as long as possible. The breaking point seems to be 55. After that women seem more willing to accept it, though we suspect that that point will be pushed steadily back to 60 and beyond."

Antonia Barber, a successful children's book writer, is typical in many ways of those who look younger than their years, though she does not fit all the criteria. She enjoys exercise, spends 20 minutes each morning stretching and bending, walks her dog every day and looks after a two- acre garden at her home in Kent.

She was also very happily married, until her husband was killed in a car crash 12 years ago, and mixes with people much younger than herself. "I was comparatively old when I started a family. We adopted three children, the first when I was 35, so I was catapulted into the company of their friends' parents, who tended to be 10 years younger."

But her husband was not younger than she (one year older, in fact) and she has never used HRT. "My doctor suggested I should do so to avoid osteoporosis [brittle bones], so I had a bone scan to see if it was necessary. In fact, although I was already seven years past the menopause, my bones had deteriorated very little.

"I am against taking medicines, if I can avoid it, though I do go to the health shop for garlic, mineral or vitamin pills if I am feeling a bit run down or depressed. I eat a good diet and have never smoked. Above all, I really love life."

Dr Weeks is also studying a group of people who say that they feel younger than their chronological age. Some of these overlap with the group that also looks younger, but Dr Weeks and his team have analysed their qualities separately. Their most common traits include having a wide range of interests, seeking novelty, challenges and stimulation, being enthusiastic and curious and having a sense of humour.

"They have a playful attitude to ideas," he adds.

A good example of this group is Jane Lander, a 63-year-old widowed physiotherapist from Birmingham, whose interests include rambling, fell walking, keep- fit classes and genealogy. She went on an outward bound course with a group of 30-year-olds two years ago and has just started a long-distance learning course in theology at Oxford University.

"I both feel and look young for my age. It seems to run in the family. One of my daughters got on the bus one day, when she already had three children, and was asked if she wanted a half fare.

"I do mix with people younger than me, because many of the other physiotherapists in the hospital and hospice where I work are much younger, but my husband, who was a blind physiotherapist, was seven years older."

Dr Weeks has found that both groups tend to have less cardiovascular disease than the general population (possibly connected to their high rates of exercise and low rates of smoking), but he has not yet discovered any other common health factors.

Some of the volunteers find it annoying if doctors dismiss their complaints as part of the normal ageing process. One went to her GP with a pain in her right knee. The doctor told her it was just her age. "That's strange," she said. "There's no pain in my left knee, and I have had that one just as long."

What they all seem to have in common is an insatiable interest in life. They would agree with the French writer Andr Maurois, who said: "Growing old is a bad habit which a busy man has no time to form."