H’s aunt did crossword puzzles “like it’s her job,” as the family said. She did develop dementia in her early-mid 90’s. Maybe the puzzles staved off the inevitable. Maybe not. But she enjoyed doing them.
I love doing all sorts of puzzles - crossword, Sudoku, NYT games, games on/against my computer (Scrabble, Free Cell, Spider Solitaire). I just really like figuring things out. Whether or not it keeps me from getting dementia is irrelevant for me. It is something that makes me happy.