Agreed 100% on the importance of magnesium. I've been supplementing with magnesium citrate since I discovered that magnesium 'oil' (really a 'simple syrup' of water and magnesium chloride, which feels oily) worked on the leg cramps I was getting. As an open water swimmer who cannot float, cramps are... not good. There are multiple kinds of magnesium available as supplements; citrate hit the sweet spot on cost / bioavailability. Oxide doesn't seem to be absorbed at all, sulfide (Epsom) is
That said, and back to staving off dementia, there's potentially some evidence that l-serine might also help. It's somewhat mixed in the sense that the people in one of the 'blue zones' in Japan get a lot of it in their diets - but autopsies of people who died with AD don't show markedly less. My n=1 (anecdata) is that phosphodytlserine a few hours before bedtime helps me to go to sleep when I'm out of synch (jet lag).