I think it's somewhat ironic that TB's are raced at 2, nowhere near mature, snapping bones, and ruined for life due to early stress on their joints and bones, and it's just fine..
But breed a horse, especially one that's younger, and OMG-- those panties bunch up a wad the size of Kansas.
Wadded panties notwithstanding, whatever decisions breeders make with regard to THEIR horses depends entirely on the horse. Each one has to be assessed individually.
Breeding is an industry based on the ethics and morals of being aware of what you are creating genetically. You are speaking of training in your reference to TBs - whether training happens at two, three or ten years of age has no bearing or relevance on whether or not a breeder chooses to breed a horse at two or five - it's not the breeding horse that gets hurt, it's the potential offspring if the breeder has not done their homework.