So the UK is ageing. But the UK is not alone. Population ageing is a global challenge. Over the next 50 years, the age composition of nearly every country is expected to move to one in which the old outnumber the young. In 20 years' time half the population of Europe will be aged over 50. So the recent announcement from the Office for National Statistics that the UK now has more pensioners than children under 16 simply places us in line with world trends.

The main cause is falling fertility. Current UN population forecasts suggest that Europe will hover just below the replacement level of 2.2 children for at least the next 50 years. However, longevity is also increasing. When UK state pensions were introduced in 1948, average life expectancy for male manual workers retiring at 65 was 69. Now a 65-year-old man can expect to live well into his eighties.

We are seeing a fundamental shift in the demographic structure of society. It will affect the way we live, the way we work, public services and healthcare, private and public benefit systems, families, communities, patterns of saving and consumption, provision of housing and transport, our education systems and even the geopolitical order of the 21st century.

The UK labour market, for example, is being transformed through population ageing. For several decades now, the UK has relied on young migrant labour to compensate for its ageing population. But as fertility falls across Asia and Latin America, global ageing will intensify the world skills shortage and potentially create severe competition in the labour market. It is essential that the UK looks to the large skills base it has within its own economy, and retains experienced older workers in their fifties and sixties. Such a policy will also address concern over increased spending on pensions, as people will work and contribute to the pension pot for longer. This will allow more of the public purse to be spent on the growing number of over-eighties, who will need long-term care.

One employer, early promotion and early retirement are likely to be replaced by flexible working, frequent employment moves or even portfolio careers with regular retraining and changes in occupation. As life-long learning and adult education become more widespread, mid-life company directors may retrain as teachers; shop assistants upskill to become company directors. These working lives will be punctuated by employment breaks to allow for child rearing, travel, education, elder care, self-employment and leisure. Men and women in their fifties and sixties, whose children have left home, may increasingly join the growing number of UK migrant workers who will travel oversees to Asia or the Americas for a few years to take up the opportunities of the international labour market. Retirement is likely to move from an abrupt halt at a fixed age to a more gradual withdrawal, with the abolition of compulsory retirement ages. UK workplaces will benefit from age-integrated workforces that include both young and old workers.

Our cities will change too. The increasing dominance of the UK high street by retailers targeting the young will be replaced by more age- segregated shopping areas. The district of Sugamo in Tokyo, for example, is now a vibrant area for fashion and entertainment for the over-sixties, which more than mirrors the city's Harajuku District, aimed at Tokyo's teenagers. Indeed, this is the future UK retail market. The over-fifties hold 60 per cent of the UK's savings, represent about 80 per cent of the UK's disposable wealth and are responsible for 40 per cent of the consumer demand in this country, spending some £200 billion a year. In marketing circles the “grey market” is set to become the new “black”. About half the fifties regularly shop on eBbay and more shop online than the under-thirties. Advertisers are beginning to recognise the over- fifties as creative consumers, who respond as well to attractive relevant marketing propositions as their children and grandchildren do.

UK families are changing too. Falling fertility and increased longevity is leading to multi-generational families - more grandparents and grandchildren, more great- grandparents and great grandchildren, but fewer siblings, cousins, aunts, uncles, nieces and nephews - the “beanpole family”. We are also now seeing a rapid growth in extended beanpole families - reconstituted step-families with large numbers of step and half-brothers and sisters. These families will provide the care for older adults of the future. But they are increasingly complex. We have grown up with the understanding of generational succession. The system works because older generations grow old and die and pass on assets, power and status to their descendants. What will happen when we have not three generations, but five all alive and active at the same time? When we are in our eighties before we inherit from our parents or even from our grandparents?

My mother's life expectancy at my age was in her mid-seventies, mine is 96. That is two decades of more life in one generation. Currently, every hour we live adds five minutes to our life expectancy. How are individuals going to restructure their lives in the light of this knowledge? During a lecture I recently gave on increasing longevity at a London boy's school, a 12-year-old piped up: “If I am going to live to 150 then I shan't have kids until I am 80!” Perhaps one of the reasons why people are delaying having children is because they realise that they will be still be around far longer to see children grow up.

The UK should not fear its coming maturity. Mature societies need not be societies of old people burdened by providing health and social care to frail elders. They provide the opportunity for age-integrated flexible workforces, increased communication between generations, age equality and political stability. As people age they accumulate a wealth of experience, knowledge, skills, memories, wisdom and creativity. Governments and employers need to work together to enable the recruitment, retention and retraining of older men and women, whose skills, expertise and experience is so valuable to our economies. Then the UK's older population will remain valued and needed members of our society. The UK's young need not fear the future either. In a future world of scarce skilled labour, they are a valuable commodity - with a huge international labour market eager to attract scarce skilled workers. Mature societies provide the opportunity for multi-generations to live and work alongside each other, contributing their own experiences and expertise. This is successful population ageing.

Sarah Harper is Professor of Gerontology at Oxford and director of the Oxford Institute of Ageing

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