Jumping young horses?

In my opinion a horse should not be jumping any serious height until their joints have fused, the age of this will vary from horse to horse. If they are jumped hard before this time then there is a chance of permanent damage being caused, some horses will be lucky and be fine but you can't tell which ones will suffer and which won't so the same rule should apply to all horses. A vet exam will confirm if the joints are fused or not so there is no excuse for not knowing or guessing.