LONDON (Reuters) - Being intelligent is no guarantee of a happy old age, researchers said on Friday.

They studied 550 people born in Scotland in 1921 whose mental ability had been tested at age 11 and again at 80 to determine if intelligence over a lifetime was linked to happiness.

"In older people there seems to be no relationship between how well they do on tests of their mental ability and thinking memory skills and how satisfied they are with their life," said Alan Gow, of the University of Edinburgh in Scotland.

The study, which is published in the British Medical Journal, looked at how people maintain their mental ability as they age and the impact it has on their lives in an effort to discover the secret of successful aging.

In addition to the intelligence tests, the elderly people completed a satisfaction with life survey in which they rated how content they were. The researchers found no relationship between satisfaction scores and cognitive ability.

While not linked to happiness, intelligence does have an impact on how people function in old age, the study showed.

"With an aging population it is important to know what causes people to age successfully. What it is about some people that allows them to have a full and independent old age. One of the things that allows people to live independently is maintaining their cognitive ability," said Gow.

The     United Nations estimates that by 2050 the number of people over 60 years old will reach 2 billion.

The scientists are continuing the study to determine what factors in the peoples' lives may contribute to happiness.

SOURCE: British Medical Journal July 16, 2005.

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