I feel that the age of five is a good age to start learning about how to play chess. Most of my own children started playing chess at the age of five and were able to grasp the concept of the game by that time. As far as when to start playing in the chess club, that's a different story.
It's my opinion that second grade is the best for children to join chess club. I feel that kindergarten is too early for children to join the chess club and don't offer it to families at that age. I have made exceptions in the past but those exceptions are far and few between. Social maturity is the big issue for me with children at that age.
I open the chess club for children in first grade but I feel that is still a little too early for children as well. Although first graders can understand how to play chess they seem to really struggle in the chess club. They lose many games and seem to get frustrated by losing so often. I also spend most of my time with the youngest children in chess club which gives me less time to work with others who may need it.
Second grade is the perfect grade for a child to start chess club. It is at that time that children really seem to grasp the concept of the game and are mature enough to not take losing as personally.