"These changes are different and separate from the changes in thinking skills that occur as people get older," said study author Valgeir Thorvaldsson, MSc, of Goteberg University in Sweden.
"We found accelerated changes in people's mental skills that indicated a terminal decline phase years before death," Thorvaldsson added.
The start of the decline is different for various cognitive abilities.
Perceptual speed, which measures how quickly people can compare figures, begins declining nearly 15 years before death.
Spatial ability starts declining nearly eight years before death. And verbal ability starts declining about six-and-a-half years before death.
The study involved 288 people with no dementia who were followed from age 70 to death, with an average age at death of 84. The participants' mental skills were measured up to 12 times over a period of 30 years, and they were evaluated to make sure they had not developed dementia.
A number of factors may explain this terminal decline in mental skills, Thorvaldsson said.
"Cardiovascular conditions such as heart disease or dementia that is too early to be detected could be factors," he said.
"Increased health problems and frailty in old age often lead to inactivity, and this lack of exercise and mental stimulation could accelerate mental decline," he added.
Thorvaldsson noted that verbal abilities declined sharply in the terminal phase and did not decline significantly due to age only.
"This indicates that people remain stable in their verbal abilities unless they are experiencing disease processes that also increase their mortality risk," he said.
"A change in verbal ability might therefore be considered a critical marker for degeneration in health in older people," he added.