Decline in cognitive brain function may begin in the mid-40s - up to 15 years earlier than previously thought, new research suggests. But that may not be the whole picture, Channel 4 News hears.

Please wait while this video loads. If it doesn't load after a few seconds you may need to have Adobe Flash installed.

The brain's capacity for memory, reasoning and comprehension begins to wane much earlier than previously thought, research published in the British Medical Journal suggests.

But one expert on ageing has told Channel 4 News people must not jump to conclusions about the findings.

While 60 has been cited as the age people experience a difference in their thinking abilities, the new study suggests that the decline could actually start at 45.

But Professor Christina Victor from Brunel University's Institute of Ageing Studies told Channel 4 News: "There are good things about the study but there are also caveats about the inferences we can draw from it."

These tests are really very abstract and they require you to do artificial things in artificial circumstances. Professor Christina Victor

Researchers from the Centre for Research in Epidemiology and Population Health in France and University College London in the UK studied more than 7,000 Whitehall civil servants over a 10-year period.

The research focused on a sample aged between 45 and 70 at the start of cognitive testing, in 1997 to 1999.

Cognitive function was measured three times over 10 years to assess memory, vocabulary, hearing and visual comprehension skills.

Encouraging

Professor Victor, who is professor of gerontology and public health at Brunel, told Channel 4 News she was encouraged by the research but pointed out that given the nature of the tests, it is unclear how the findings may apply to our daily lives.

She said: "What does a 3.6 per cent decline mean in our ability to function in the community? These are tests which are done in a lab or interview setting but it's not clear how this applies to everyday life.

"I think there are a lot of questions. For example the group studied are all civil servants, so you have to think is that generalisable across the rest of the population? These tests are really very abstract and they require you to do artificial things in artificial circumstances."

But she said generally the results could be seen as positive.

"For me the encouraging thing about the research is the link of decline to lifestyle factors. The study gives hope that we may be able to reduce or prevent dementia by management of our lifestyle," she said.

Since the study looked at participants aged 45 and over, it is not clear from the findings if there may be measurable decline even earlier.

As Professor Victor pointed out, some physiological parameters such as language acquisition ability peak at a very early age, and the paper itself states "the age at which cognitive decline begins remains unknown" so before consigning ourselves to the psychological scrap heap, we should wait perhaps for yet more research.