Yet another study shows that a large fraction of retired professional football players have sign of traumatic brain injury. Most previous studies had relied on diagnoses that could only be made postmortem, but this one use an MRI technique called diffusion tensor imaging. Other studies (the postmortem type) appear to show that any level of participation, high school, college or professional produces brain injury.
These results apply to American Football, but other studies show significant risks from rugby and soccer. In soccer, the principle risk appears to be related to heading the ball, which tends to produce not only hard collisions with the ball but also with other player's heads and elbows.
Children, at least, should probably never head the ball. This kind of rule is unlikely to compromise their development as players much, since head skills are relatively simple compared to foot skills.