Terry is interested in slowing the cognitive decline that comes with aging. according to current research, what is the most beneficial change he can make?

The answer is "increase physical exercise".

Various studies in the previous couple of years have given confirmation that physical exercise can enhance the impacts of aging on the mind, in terms both of avoiding or delaying dementia, and diminishing the more ordinary age-related decrease in cognitive function. While the subjective advantages of physical exercise for kids and more youthful grown-ups are less clear, there is some proof that there might be some advantage, despite the fact that not to indistinguishable degree from for older adults.