Cognitive functions like memory, reasoning and comprehension skills start falling off at the age of 45, a new study including Indian origin researcher has revealed. Previous research suggests that cognitive decline does not begin before the age of 60, but this view is not universally accepted. Researchers, led by Archana Singh-Manoux from the Centre for Research in Epidemiology and Population Health in France and University College London in the UK, argued that "understanding cognitive ageing will be one of the challenges of this century," especially as life expectancy continues to rise. They added that it is important to investigate the age at which cognitive decline begins because medical interventions are more likely to work when individuals first start to experience mental impairment. Therefore the authors observed 5,198 men and 2,192 women over a 10-year period from 1997. They were all civil servants aged between 45 and 70 and were part of the Whitehall II cohort study established in 1985. Participants' cognitive functions were assessed three times over the study period. Individuals were tested for memory, vocabulary and aural and visual comprehension skills. The latter include recalling in writing as many words beginning with "S" (phonemic fluency) and as many animal names (semantic fluency) as possible. The results show that cognitive scores declined in all categories (memory, reasoning, phonemic and semantic fluency) except vocabulary and there was faster decline in older people. The findings also reveal that over the 10-year study period there was a 3.6 per cent decline in mental reasoning in men aged 45-49 and a 9.6 per cent decline in those aged 65-70. The corresponding figures for women were 3.6 percent and 7.4 per cent.