Owing to rising life expectancy and healthier lifestyles, the average person no longer feels they are going downhill in their 40s, the study found.
Around 53 per cent of Britons in a survey of 2,000 men and women said they do not even believe middle age exists anymore.
Among over-50s, 43 per cent felt they had not experienced middle age yet. Nearly three quarters said there was less of a divide between the age groups than in the past, with improved health care seen as the main reason, the 'Daily Express' reported.
Around 84 per cent people said that if you think of yourself as old, you will naturally start to feel old.
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Illness and memory loss were named as top threats people fear most in growing old, according to the research commissioned by Benenden Health.
The research also came up with a familiar top 40 signs of approaching middle age. People listed frustration with modern technology as the top sign of ageing, followed by a tendency not to have a clue what young people are talking about.
"Britons are happily skipping over traditional notions of middle age. It's a term with less significance and is no longer a numerical milestone. A variety of factors are involved, including more active lifestyles," said Paul Keenan, communications chief at Benenden Health.
"It's clear that what age you are has become less important in determining how young you feel. Being old appears to be a state of mind rather than being a specific age," Keenan said.