The answer is simple - when your child has the skill set to successfully play the sport. The more successful the child is the more fun he will have and the more likely he will stick with team sports. So if your one year old can kick and control a soccer ball - get her going. . If your four year old struggles with paying attention to the rules of t-ball, wait.
I like to remind parents that the best thing you can do for your young athlete while they are on the field is to jump up and down, clap and point out everything the child is doing right. What kills sports for kids is when a parent has expectations beyond the child’s ability. And when parents make negative comments, it does nothing but have a child questioning their own ability.