The question to me is much like the pitch count controversy in that a rule can not be enforced unless a chronological age can be affixed to a ban on the pitch. Yet evaluating the problem by age is using much too dull a tool to form an reasonable opinion. This is true not only for an organization but for a parent.
The concept I would rather apply is the relative physical maturity of the individual pitcher. This requires a parent willing to learn and apply his/her common sense and determine when his athlete is mature enough to handle the unique stress latent in different types of breaking balls. The knowledge and expertise of the parent has a huge effect on that variable as does the availability of pitching coaches that are qualified to develop a young pitcher.
The overuse issue is part of the totality of circumstances in that a young pitcher becomes more likely to suffer injury when fatigued, whether by high pitch count per inning, per game, per week or per season. The breaking ball requires a conscious effort to deviate from the fastball delivery and this conscious effort will be compromised as the pitcher becomes more and more fatigued.
So IMO a parent or coach with his eye on the pitcher's mechanics, accuracy, velocity and other fatigue indicators can reduce the risk of injury and still permit the use of the breaking ball in youth pitchers. I think alot of parents and coaches allow youth pitchers to use the breaking ball for the wrong reason and fail to recognize their own limitations when it comes to the art and science of pitching.
I was not comfortable with my son throwing a breaking ball until age fifteen plus when I deemed him mature enough to handle the pitch properly and after he had developed the fastball and change up to some degree. I know he sacrificed some early success but he was a late bloomer so to speak, experiencing rapid growth spurts. Parents like JD are obviously able to manage the addition of the breaking ball at an earlier age.