Mr Wong says that growing up he always knew his age, as did his older brother, who died a few years ago. His sister, Chi Sen Wong, 72, says her mother had her at 40 and their younger brother, now in Hong Kong, at 42.

''I take more medications than my mother,'' says Mr Wong. ''I've got a pain here, a pain there, diabetes, a little bit of high blood pressure. She's not got any sickness.''

Whatever Mrs Wong's exact age, there is no doubt she is very old, though she is mobile, alert and able to converse a little with her children in dialect. Dressed in a traditional red satin Chinese jacket and her favourite cap this morning, she looks more spry than frail.

Uncertainty about Mrs Wong's age is mirrored in the wider debate over centenarians. Their exact number in Australia is unclear - 3130 in 2008, the Australian Bureau of Statistics says; somewhat fewer, more cautious demographers say. But there is no argument that many more Australians will sail past the current life expectancy to reach extreme old age.

Worldwide, centenarians will exceed 1 million in number within 20 years. Within 30 years, it is likely there will be 10 times as many centenarians, says La Trobe University's John McCormack, director of the Australian Centenarian Study.

Even baby boomers who fall short of the century mark can expect to live longer than they probably expect. Females aged 50 in 2001 have a 50 per cent chance of living beyond 90, and men a one in three chance, according to demographer Heather Booth, of the Australian National University, and actuary Leonie Tickle, of Macquarie University. They say official longevity figures are underestimates.

As for Australians born today, the official expectations are that females will live to 83.7 and males to 79. But demographers Jim Oeppen, of Cambridge University, and James Vaupel, of the Max Planck Institute for Demographic Research in Germany, say that in countries with the highest life expectancies - and Australia is near the top - girls born today have an average life expectancy of 100.

Tickle says: ''Forecasters have persistently underestimated the limits to human life.''

Frenchwoman Jeanne Calment was aged 122 years and five months when she died in August 1997. Christina Cock, the oldest ever Australian, died in 2002 aged 114.

Whether we have reached the limits of the lifespan or the increasing pace of biotechnology breakthroughs will add further to longevity, the implications of so many people living to such grand old ages are hard to grasp. It is as though longevity crept up on us.

''Indeed, many elderly people are asking in tones of weary impatience, 'How long will life go on?' '' wrote Booth and Tickle in a seminal paper in 2004, Beyond Three Score Years and Ten: Prospects for Longevity in Australia.

Since Treasury's first inter-generational report in 2002, most people understand that Australia's population is ageing because of the bulge of baby boomers, increased longevity and lower fertility rates. Paying for future pensions and health and aged care will be problematic because the working-age and tax-paying population is not growing nearly as fast as the number of retirees who will pay no tax.

Between now and 2047, it is projected that the number of people of traditional working age (15-64) will rise by about one-fifth, that of older people (65-84) will more than double, and the very old (85-plus) will more than quadruple.

Most baby boomers are now painfully aware of the need to save for retirement, even if they may underestimate the number of leisure years ahead. But what of the less easily quantifiable implications of the rise of the ''old old''? What will it be like to be a son or daughter at an advanced age when your own joints are starting to ache? When ageing parents are part of their children's lives for more than half a century, will bonds deepen or fray?

Baby-boom women were once called the sandwich generation, being squashed between caring for young children and ageing parents. But now some women hitting 60 have jobs, parents in their mid-80s to 90s, grown-up children still living at home as well as grandchildren they regularly mind for working children. They are the triple-decker sandwich generation.

The implications of inheritances delayed, of being the ''kid'' brother at, say, 90, of fathers who never retire from the family business and of the gift of unexpected years, and how they will be lived, are questions sociologists have hardly begun to ponder.

''It will be the first time in Australia we'll deal with children who are old and parents who are very old in such large numbers,'' said Colette Browning, head of the Healthy Ageing Research Unit at Monash University. Above all is the question of whether the extra years of life will be worth living.

Mrs Wong lived with her son and his wife for more than 25 years. After the death of her daughter-in-law, she moved in with her daughter until three years ago. Her children adore her, talking of her kindness, the Buddhist philosophy that guides her life, and her capacity for hard work.

''No matter how old you are, if you have a mother you are always like a kid,'' said Mr Wong, who swims regularly and plays lawn bowls. ''Even now she is always worried about me. 'Can you cook for yourself? Go eat at your sister's house,' she tells me.''

His sister, who is known as Sam, says: ''My brother and I, we're getting old. But with such an old mother we feel still young.''

Gerontologists, who study the phenomenon of ageing, are divided on issues of quality of life in the old old years. It's a matter of emphasis. The ''happy'' gerontologists - as some are dubbed - prefer to stress the positives. For example, several studies show that today's 70-year-olds are comparable to the 65-year-olds of 30 years ago, thanks to better healthcare, and most people maintain their level of mental acuity until about 70.

The Australian Longitudinal Study of Ageing, which began in 1992, concludes that many people in their 80s and older live independently in their homes with little or no assistance. ''The assumption that ageing is associated with poor health and loss of independence needs to be challenged,'' the study reports. The view that ''it all goes together when it goes'' is untrue, with many elderly able to capitalise on their strengths despite weaknesses in some areas, the study shows.

La Trobe University's John McCormack has been impressed in his work with centenarians by the ''small but growing group of remarkable survivors who, despite some frailty and adverse health conditions, still rate their own lives as worth living''. Half the centenarians in his study lived in the community, and people such as Jack Lockett, who reached 111, and Bea Riley, who lived to 112, were ''cognitively intact''.

But a darker note is sounded by others, including Jacqui Smith, a psychology professor at Michigan University who was formerly with the Max Planck Institute, where she co-wrote seminal papers with the eminent Paul Baltes - who dubbed the years 85 and beyond the ''fourth age''.

''It's important to recognise the different phases of old age,'' Smith told The Age. ''Of course, there are some exceptional individuals who continue to live independently in relatively good health to the age of 100-plus. But there are also many more who are long-lived but suffering at some level.'' The tendency to conflate the 65-plus age group into a single positive story about ''the elderly'' needs to be guarded against, given the evidence showing ''marked decreases in physical and mental health in the fourth age'', she says.

This week Alzheimer's Australia released a report by Access Economics that estimated there would be 1.1 million Australians with dementia in 2050, compared with 245,000 today, a big upwards revision of earlier estimates.

Almost 25 per cent of women aged 85 to 89 have dementia, rising to 50 per cent aged 95 or older. For men, the figures are 21 per cent and 37 per cent.

Summarising their sombre findings on the very old, Baltes and Smith said ''living a long life has its costs, medically, psychologically, socially and economically''. And questions had to be asked, they said, about whether the push to extend the lifespan will reduce ''the opportunity of an increasing number of people to live and die in dignity''.

How many of the extra years are being spent with disability and illness - and will be in future - is subject to debate. In several Western countries, the news is cheering, with falling rates of disability in people over 65. But here, official figures from the Australian Institute of Health and Welfare indicate a higher proportion of our ''end years'' is being spent in disability than in the recent past, when a massive heart attack or stroke knocked people off their perches.

Robert Long, of the institute's Population Health Unit, says that based on past trends, the number of healthy years a person can expect will grow at a slower rate than their overall life expectancy. ''Statistics keep telling us our lifespans will increase but that does not necessarily mean we will spend all those extra years in a healthy state,'' he says.

Several longitudinal studies on ageing in Australia show, unsurprisingly, that a healthy lifestyle increases the chance of a long, healthy life. As Smith says: ''It's very important to motivate young and midlife people to adopt lifestyles that allow them to experience all of the good aspects of the third age.''

Perhaps nothing is more important than giving up smoking, the studies show. After that, regular exercise - weight-bearing to strengthen bones, and stretching to reduce stiffness - is probably the single-most potent predictor of healthy longevity.

Preventing diabetes and high blood pressure is crucial. People who are socially engaged with friends, family and the community do better. Having a positive outlook seems to help.

Gerontologist Anna Howe says all Australians at age 60 should get a free comprehensive health check to set them up for healthy ageing. She attributes Australia's superior longevity record not just to lifestyle, but also to public policy measures such as Medicare, bulk-billing GPs, free public hospital care and the Pharmaceutical Benefits Scheme. And she offers this reminder: 25 per cent of Australian men die before 60 (11 per cent of women). In general, the socially and economically disadvantaged die prematurely. And with the obesity problem, a question mark hangs over the longevity of children born today.

More Australians are reaching the rocky shoals of the fourth age. But the challenge remains to open the pleasures of the third age to those who are less likely, as things stand, to get that far.