It has been said by many people that we should get knowledge wherever we are. The mind is also like a muscle, the more you use it the stronger it becomes. Just as we need physical exercises to keep our bodies fit and strong, we also need exercises for our brain. I have known a few old people who are very much active, alert and sharp in their thinking. Mind you these people are in their mid 70's and yet their brain works even better than mine.
One of the ways is to play strategic board games like chess. Of course there are online strategy games that will also help to stimulate the mind. A quick search on the net and you will see a list of strategy games that you can play. However, I know that to read books is much better than to play games. Reading books will expand your mind to other thoughts and imaginations that you would never thought of on your own. It is more like reading the minds of other. Whatever you choose to do, be it play a game of chess, online game, reading a book or start an e-learning course, the point is that keep your mind active and alive. If you do that then hopefully you will keep Alzhemier away.