How much should children participate in sports?

Obviously, it's good for kids to exercise, and there are plenty of psychological and social advantages to kids participating in sports, but I've been reading a lot lately about how children cause permanent damage to their knees and legs playing sports too often because their joints, muscles, and bones are still growing. This article, for example, says that 20% of young athletes will get Osgood-Schlatter syndrome (when tight muscles pull a growth plate apart).


So, is there a limit to how much sports kids should play? What age is it healthy for kids to start practicing harder and training for potential pro careers?