But to keep it simple: read, do puzzles, stay active, do not drink or do drugs heavily. ——
Everybody already has a genetic makeup that limits their health—especially the brain—to a preset level. Some people suffer from infantile amnesia—basically not remembering early developmental stages (2-4) and preadolescent (~10 y/o); other’s claim to remember their birth (which is silly since ability to form memories is 2 y/o+). Some have the luxury of having strong working memory^1—a fancy word for temporary memories—but suffers from lackluster short-term memory.
I’m sure you get the point I was trying to make with that mediocre explanation.
tldr: You can not change your brain to not have dementia if your genetic makeup says so. You can however delay the onset possibly by keeping your brain active on a daily basis.