The mental skills of the elderly begin to deteriorate independent of both age and dementia(痴呆), rapidly accelerating as much as 15 years before death. Previously, investigators believed that this sort of decline began just three to five years before death, but a study of 288 elderly people found that verbal ability, spatial reasoning, and perceptual speed begin deteriorating years earlier. Insufficient exercise and mental stimulation in old age and a number of health factors may trigger this rapid loss of cognitive(認識の)skills.