Collecting the state pension and bus pass at 65 has traditionally been seen as a watershed moment where middle age ends and the twilight years begin.

But new research suggests that old age now starts at 74, with middle age lasting at least nine years longer than current estimates.

Academics from the International Institute for Applied Systems Analysis (IIASA) in Vienna, Austria, argue that old age should be measured not by age, but by how long people have left to live.

In the 1950s a 65-year-old in Britain could expect to live a further 15 years.

But today’s baby boomers are expected to live far longer after retirement. A recent estimate by the Office for National Statistics suggests that the average retiree can look forward to drawing their pension for up to 24 years – as much as 50 per cent longer than their parents’ generation.

Researchers say that old age should be defined as having 15 or fewer years left to live, which for the baby boomers means that they are still middle aged until their 74th year.

“If you don't consider people old just because they reached age 65 but instead take into account how long they have left to live, then the faster the increase in life expectancy, the less aging is actually going on,” said Sergei Scherbov, World Population Program Deputy Director, at IIASA.