There is a strong correlation between exercise - or level of physical activity - and the pace at which the brain declines. One of the plausible connecting mechanisms is the quality of blood vessels and level of blood flow in the brain, both of which suffer in sedentary people. Other possible candidates include the pace at which new brain cells are created, and the pace of decline in the portions of the immune system that support brain tissue. Both of those are also impacted by exercise.
Unfortunately the downward spiral of degenerative aging is compounded by the fact that exercise and eventually any meaningful physical activity become increasingly hard to undertake. The erosion of muscle mass and strength, and rising frailty due to other causes, can accelerate the decline of mental capacity by indirectly attacking the physical foundations of the brain's operation. All of our biological systems are intertwined, after all.
Equally, correlations between brain and body aging can be looked upon as the result of a general decline in robustness: if there is more cellular and molecular damage throughout the body, then you would expect to see that, on average, greater physical and cognitive decline occur together in the same individual. More damage means a greater loss of function.
Here is a recent paper that touches on the strong relationship between frailty, exercise, and cognitive decline:
The 90+ Study is a population-based, longitudinal, epidemiologic study of aging and dementia performed at the University of California, Irvine, from January 1, 2003, through November 30, 2009. .., A total of 629 participants from The 90+ Study were included in the study. The mean age was 94 years, and most (72.5%) were women.
All-cause dementia [was] was the main outcome measure. ... Odds of dementia in relation to the physical performance measures were estimated by logistic regression after adjustment for age and sex.
Poor physical performance in all measures was significantly associated with increased odds of dementia. ... We found a strong cross-sectional relationship between poor physical performance and dementia in people 90 years and older.
You might also look at the paper that was referenced in one of yesterday's posts, in which all sorts of physical changes in the aging brain are less progressed in people who exercise more. A great many similar studies have amassed over the years.
Increased participation in leisure and physical activities may be cognitively protective. Whether activity might protect the integrity of the brain's white matter, or reduce atrophy and white matter lesion (WML) load, was examined in the Lothian Birth Cohort 1936, a longitudinal study of aging.
Associations are presented between self-reported leisure and physical activity at age 70 years and structural brain biomarkers at 73 years. For white matter integrity, principal components analysis of 12 major tracts produced general factors for fractional anisotropy (FA) and mean diffusivity. Atrophy, gray and normal-appearing white matter (NAWM) volumes, and WML load were assessed.
A higher level of physical activity was associated with higher FA, larger gray and NAWM volumes, less atrophy, and lower WML load. The physical activity associations with atrophy, gray matter, and WML remained significant after adjustment for covariates, including age, social class, and health status.