Chess is great for seniors because it is great for everyone. Aside from being a fascinating and complex game, chess is accessible. All you need is a set and another player, or a device and an internet connection.
Judit Polgár had two chess goals after reaching GM standing. One, she wanted to be ranked in the top ten in the world. Two, she wanted a shot at the world title. She achieved one and got very close to the other.
