Parents have been warned about the dangers of their children heading a football.
A leading doctor said the impact can damage neck muscles and cause brain injuries.
Dr Michael Grey, reader in Motor Neuroscience at the University of Birmingham's School of Sport, Exercise and Rehabilitation Sciences, said repeatedly heading a ball affects a child's neck muscles that are not yet fully developed and the brain is still immature meaning it is more vulnerable to damage.
Dr Grey said: "I do not think that children should stop sport, the obesity epidemic means we need to encourage them.
"But we do need to look at rule changes and the way we train children.
"Children should not be heading the ball. We don't know at what age children's necks become strong enough to withstand the movement of the head when the head is struck by the ball.
"Some of my colleagues have suggested 14 but I really think it is individual.
"In addition the brain starts to shake and rotates when the head is struck by the ball.
"The brain bounces back and forth and it is the impact of the brain against the inside of the skull causes additional damage."
Dr Grey added: "There is a considerable amount of newer evidence coming through showing the need to be concerned.
"Because it is such a new topic in the UK word really hasn't filtered out properly to the public.
"Our challenge is that because it is such a difficult issue. We know some people are very susceptible to getting head injuries in these situations and some are quite resilient and we don't know why yet.
"We need to properly educate parents, coaches, referees about secondary concussion syndrome, we need to invest in research but the biggest thing we need to do is follow the existing guidelines, which include a test for players.
"We need to develop good tests based on physiology not behaviour so it is not possible for players to cheat. "In essence we need to protect players from themselves."