The brain's ability to function can start to deteriorate as early as 45, a study in the British Medical Journal suggests.
University College London researchers found a 3.6% decline in mental reasoning in people aged 45-49.
They assessed the memory, vocabulary and comprehension skills of 7,000 men and women aged 45 to 70 over 10 years.
The Alzheimer's Society said research was needed into how changes in the brain could help dementia diagnoses.
Previous research had suggested that cognitive decline does not begin much before the age of 60. But the results of this study show that it could begin in middle age.
This is important, the researchers say, because dementia treatments are more likely to work at the time when individuals start to experience mental impairment.
Individuals were tested for memory, vocabulary and aural and visual comprehension skills.
Differences in education level were taken into account.
The results of the UCL tests show that cognitive scores declined in all categories except vocabulary - and there was a faster decline in older people.
The study found a 9.6% decline in mental reasoning in men aged 65-70 and a 7.4% decline for women of the same age.
Professor Archana Singh-Manoux from the Centre for Research in Epidemiology and Population Health in France, who led the research team at University College London, said the evidence from the study showed that dementia involved cognitive decline over two to three decades.