The United Kingdom is officially a grey nation. The number of pensioners exceeds the number of children under 16 for the first time, figures published yesterday show. The milestone, reached last year and revealed now by the Office for National Statistics, is both a cause for celebration and for consternation.

The ageing of the population is a tribute to the success of the modern world in raising living standards, improving healthcare and extending life expectancy. A boy born in 2005-07 can expect to live for 77.2 years, six more than one born in 1980-82, and a girl can expect to live for 81.5 years, four longer than in 1980-82.

But when allowance is made for further expected improvements in mortality during their lifetime, even these advanced ages understate the real length of likely survival. On this basis, the average projected life expectancy (known as the cohort life expectancy) for babies born in 2006 was 88.1 for boys and a breathtaking 91.5 for girls.

That's the good news. The bad news is that the rapid growth in the very old will place an increasing burden on the young, who must support and care for them. In the 25 years since 1982, the number of over-85s has more than doubled to 1.3 million and will more than double again to 3.1 million in the next 25 years.

Older people are the fastest growing group, with the over-65s numbering 9.8 million in 2007 and projected to increase to 16.1 million by 2032, almost one in four of the population. This will impose heavy demands. About 40 per cent of NHS spending goes on the over-65s, who comprise 16 per cent of the population. Pensioners get more than £60bn in cash and benefits.

Advances in healthcare mean people are living longer but not necessarily in good health. On average, a man aged 65 in 2004-06 could look forward to 10 years of healthy life and a woman seven months more. Almost half of 65- to 74-year-olds have a long-standing illness that limits their activities and this rises to more than 70 per cent among the over-85s.

As the population ages, so will the workforce. The Government has plans to increase the pension age for women from 60 to 65 between 2010 and 2020 and for everyone from 65 to 68 between 2024 and 2046.

But it would be wrong to be gloomy. The successes of the past 40 years have made premature deaths for men fall by 38 per cent and for women by 29 per cent. The biggest improvement has been in heart disease: deaths there have plummeted by more than 50 per cent, largely driven by the decline in smoking. "Improvements in mortality rates are likely to drive the growth in numbers of older people for the next 25 years at least," says Karen Dunnell, the ONS national statistician, in her second annual article on the population, Ageing and Mortality in the UK.

Will we become a nation of dodderers? That depends on how we age.