Unlike some diseases, nobody really know WHAT causes dementia!
Didn’t Mrs. Reagan believe that something about the assassination attempt may have had something to do with Reagan’s dementia?
Of course there is no way to know what causes it in every case. But the people I’ve known who have kept their minds very active have done pretty well into great old age. (Jacques Barzun, for example, remained in his good, right mind until his death at 104. He published his last work at the age of 97.)
https://en.wikipedia.org/wiki/Jacques_Barzun