At a coaching course I attended we also discussed this. One thought is that while an age limit does not stop the training of big skills, it is likely to effect the type of training in relation to the skills. For example, if you have a young gymnast training a double back. If they compete it then they will do those many many repititions on a competition surface. If they are not permitted to compete it then the training is more likely to be to mats or the coach may spend more time training the skill on the fast track.
Perhaps a better example would be a front tuck on beam. If the gymnast is not allowed to compete it then the training is more likely to use a pad or mat over the beam. If the gymnast can compete it she will do many many more repititions onto the harder competition surface - much more likely to cause the heel and foot injuries.
In the UK there are many restrictions on skills up to Junior elite level so the rules ban skills that are best left to more developed bodies. The compulsory coaching qualifications also guarantee that the coach will have heard the BG advice, even if they choose to ignore it.
Personally I think a combination of coach education and age limits/skill bans at younger age groups is the best way to prevent burnt out, injured gymnasts.
