Why is it that some people survive to 90+ without significant mental deterioration? My grandfather is 94 and reads the news, does crosswords and sudoku, and pla

Why is it that some people survive to 90+ without significant mental deterioration? My grandfather is 94 and reads the news, does crosswords and sudoku, and plays poker and bridge, at a level comparable to when he was 60. His knees have deteriorated so he stopped jogging 20 years ago, but he still walks and swims. It's worth pursuing longevity science if we can help more people age this way.