When I talked to a pediatrician some years back about my kids doing judo, which included Aikido, and jujutsu btw, he recommended kids wait until they are 20 years ideally. He said that when most males stop growing at around that age.Their joints are fully formed and durable to take the stress and resist injury better. He mentioned the new Olympic age rule for girls to compete in Women Gymnastic because of the mental stress what hurting them, the toll it took physically in injuries resulted in a older competing age. He said, why do I want my kids to put their bodies at an added risk at this young age, what is the rush? Do you really want to see me more often, than you do now?
All doctor don't share the same opinion. I am passing on what mine said. I think the best advice is to talk to an Othopedic specialist, as well as other speicalist to get a full education on the subject. Then decide for yourself. After all your kids welfare dependents on you the parent.
If we all followed this advice our children would be kept from doing anything physical in their early developmental stages and would end up with physical, sensory integration, and mental development delays. Children have developing proprioception and vestibular systems that need to be challenged. Not to mention muscular development and movement patterns that need to be formed and integrated. This idea that keeping children stagnant keeps them safe is ridiculous. Kids NEED to get out and move and they need to move in a wide variety of ways that take full advantage of their available ranges of motion.
Thousands of kids play football, baseball, soccer, hockey and a whole host of other sports with no adverse effects. Yes there are injuries but with proper instruction, age specific rules, and competent coaching and refereeing most kids should be able to participate in sports and expect a pretty reasonable level of safety. I see the bigger problem being immature, over competitive parents who push their children beyond what is safe or reasonable.
Just my take.