According to leading British gerontologist Professor Tom Kirkwood, ageing is probably due to the gradual and progressive accumulation of damage in the cells and tissues of our bodies - as opposed to a pre-programmed formula still preferred by some ageing specialists.

In his book Time of Our Lives , Professor Kirkwood, who is head of biological gerontology at Manchester University , argues that while we are genetically designed to put huge effort into the upkeep of the germ-line - egg and sperm cells - which will pass on our characteristics to future generations, our genes pay less heed to the repair and maintenance of the somatic cells which go into heart, lungs, kidneys, brain and the rest of the body.2At a species level, the effect can be seen in the vastly differing life spans of animals. The life of the rat and mouse is short, but hugely productive in genetic terms since these animals have frequent large families of offspring to carry on their blood line. In contrast, primates reproduce rarely and have small families but reach ripe old age.

Ageing is likely to happen because genes treat organisms as disposable: they invest enough in maintenance to enable the organism to get through its natural expectation of life in the wild environment in good shape, but more than this is a waste, writes Professor Kirkwood.

Somatic cells have a finite lifespan and, in older people, these lives get shorter and shorter. Senescent cells begin to appear in the skin and other tissues. This could be due to accumulated cell damage, to the flicking of genetic switches or, as Professor Kirkwood suggests, to an interaction of the two systems.

Random damage may activate genetic control mechanisms, with names like senescence-derived inhibitor and mortality factor 4, which suppress cell division in an effort to protect the body from further damage. But this signals a gradual drawing down of the shutters for the ageing body.

Those who make it to extreme old age in good health may be those who have somehow managed to avoid accumulating too much junk in their cells and triggering the control mechanisms that lead to cellular ageing.

They may have more accurate mechanisms than most for making the vast array of proteins which the body needs for routine functions and for clearing up the garbage from everyday metabolism - so These chemical relatives of oxygen are released when cells use oxygen to produce energy , but left unchecked they can wreak havoc around cells.

Companies specialising in anti-ageing products already make millions of pounds from marketing anti-oxidant products which claim

to mop up free radicals. Eating fruit and vegetables rich in the antioxidant vitamins C and E may also help. But centenarians may have particularly well developed, natural anti-oxidant systems.

They must also have a range of genes which reduces their susceptibility to the most common enemies of the ageing body - heart disease, strokes and Alzheimer s disease. As yet, only one of these susceptibility genes - for Alzheimers disease - has been elucidated, and shown to have a favourable profile in those who reach extreme old age.3But candidates are under investigation for heart disease, cancer and other important killers of elderly people.

Identification of these mechanisms brings the potential for therapeutic interventions which could offer the prospect of extreme old age for all.