Researchers studied nearly 7,400 British people for a decade and for those aged 45-49, it was discovered their ability to reason declined 3.6 percent in the time they were studied.
The decline sped up for people aged 65-70: men experienced a 9.6 percent decline in ability to reason, and a 7.4 percent decline for women.
Other mental abilities that faded included memory, and so-called verbal fluency, which measures a person's ability to quickly say words in a particular category. However, people's vocabulary didn't change.The BMJ report says that it was previously widely accepted that cognitive decline did not occur until age 60.
The researchers say that with increasing life expectancy, understanding cognitive aging will be a challenge of this century.