My late Ah Kong lived till he was 92... He was an avid reader of books and papers, and he would also went for his daily walks in the morning and evening.. He always told me that reading keeps the mind alert.. And on top of that he would always have some brandy once a week...
my mum is 95 and her mind is sharper than mine. I believe it has a lot to do with actively using her mind everyday. For decades, she had been doing marketing and planning the house meals without the aid of pencil and paper. Everything was planned and memorized in her head.