They said that, contrary to the common perception that people of that age need to be cared for in a nursing home, a "significant proportion" of nonagenarians are free of dementia.

Others do have some memory problems, but can continue to live on their own, they say.

Researchers visited over 100 nonagenarians at home to conduct neurological and cognitive tests.

It is important to stress that there is a huge difference between 'senior moments' and Alzheimer's Research into Ageing spokeswoman

They were able to identify those who had normal cognitive abilities, those who had mild cognitive impairment and those who had dementia.

Dr Bradley Boeve, a neurologist who led the research at the Mayo Clinic, Rochester, Minnesota, said: "Just because you're in your 90s does not mean you'll be living in a nursing home or developing dementia.

"While there may be some decline in cognitive performance with age, dementia or Alzheimer's disease are not inevitable in all those living well beyond 90 years of age.

"At least half of these people looked pretty normal."

'Genetic elite'

A spokeswoman for the charity Research into Ageing, part of Help the Aged, told BBC News Online the study was good news.

She said: "It helps to show that getting older doesn't automatically mean severe illness or dependency.

"More of us will have the chance of living through into our 90s than ever before so improving our understanding of health in old age is vital.

"People rightly dread the idea of dementia so it is important to stress that there is a huge difference between 'senior moments' and Alzheimer's.

"We need to tackle conditions like Alzheimer's which affects one person in five over the age of 80, but reversing that statistic shows that four out of five over 80s are unaffected."

She added: "It is also worth noting that people who live through into their 90's are effectively the genetic elite.

"Genes account for roughly 25% of the factors that influence longevity. So if you get to 90 plus you've probably got good genes and so are likely to continue to age well."

The research is published in the journal Neurology.