Living to 120 may be a pleasant experience that is relatively free from disease, not a chore of enduring decades of ill-health, a leading geriatric psychiatrist suggested this week.

The theory is a sign of new thinking among gerontologists that longevity is no longer to be feared.

Speaking before a conference on dementia, Robin Eastwood, professor of psychiatry at St Louis University medical school in Missouri, said: "People are wondering whether heart disease, cancer and Alzheimer's may be middle-aged diseases and if you survive them you may live on to a frail but cognitively intact old age," he said. "Until this moment we have been thinking all this old age is terrible but it ain't necessarily so. The only real problem is money."

A study of a 121-year-old French woman, who is mentally alert, is helping to provoke the theory that there are few new diseases to face once the big three have been successfully avoided, he said.

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The new thinking is the result of several strands of research that are disentangling the normal effects of ageing from the development of Alzheimer's disease. Professor Eastwood, who chaired the conference held by The Lancet, said: "We know people start to develop Alzheimer's at about 65. But if you look at populations who are about 100 years old not everyone has it."

Meanwhile, researchers are trying to find the environmental triggers for the disease after a genetic predisposition was discovered several years ago. One finding has been that being intelligent or well-educated is protective.

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Professor Eastwood has set up a study of Jesuit scholars at St Louis. Jesuits are renowned for their extensive and continuous education. The pilot study of 200 scholars with an average age of 63 aims to discover their rates of Alzheimer's.

The study could eventually be extended to the 2000 Jesuits in the United States.