Why I... believe we are not prepared for the old-age revolution

We are only just beginning to understand the science of human ageing. I have been in the field for 25 years, but I realised quite early on that the conventional view we have of the ageing process - that ageing is somehow programmed into us - is wrong.

We are programmed to survive. Ageing comes about through the accumulation of faults and damage in the cells and tissues of our bodies, and we can affect that process if we can find ways of reducing our exposure to damage or enhancing the maintenance and repair systems. In principle, the ageing process is significantly malleable.

We are experiencing an unprecedented revolution in longevity, which is changing the structure of societies and requires that we revise our attitudes towards ageing. This revolution has left us ambivalent about what has been achieved. It is no longer special to be old; we have rather negative and deeply ingrained attitudes towards ageing. Without thinking, we do much that marginalises and demeans the status of older people.

One of the most important things that has to change is our attitude to the active participation of older people in society, including their continuing to be economically productive. Retirement at 65 was introduced by Bismarck when he was chancellor of Germany and life expectancy was probably not much more than 50 years. The idea was that people could retire at 65 and have a short period to set their affairs in order before they died.

Now that people are reaching "conventional" old age in better shape, quite a number find that when the age for retirement comes along they want to continue working. Not necessarily full-time - but they do not want to be locked out of the jobs market.

We see the situation most clearly in the developed West, but demographic changes are coming worldwide. By the middle of the century, it is likely that 20 per cent of the world's population will be 65 or older. The major drivers of increases in life expectancy have been in public health - clean water, vaccines, antibiotics and such. These are available for deployment in developing countries, so it is not as if they are waiting for breakthroughs.

Tom Kirkwood is giving this year's Reith Lectures, "The End of Age", on Radio 4 (Wednesdays 8pm, repeated Saturdays 10.15pm) until May 2.

Tom Kirkwood is Head of the department of gerontology, Newcastle University.

• Interview by John Davies