One of the things that ethical trainers struggle with in getting two year olds to the track is when they're old enough or physically mature enough to breeze.
One trainer I knew would xray knees to determine when the growth plates closed and wouldn't breeze before then. Another swam and jogged his babies once they were broken rather than gallop the traditional mile to mile and half; and didn't start serious conditioning, let alone breezing until the growth plates were closed.
I would love to see a program at the track where you couldn't work from the gate or work on the rail in the mornings without a vet exam certifying the horse's growth plates were closed. This would also reduce the incidence of bucked shins, because by the time the growth plates closed, the periosteum would be more mature as well and less prone to microfractures. (That's what bucked shins are, microfractures of the periosteum on anterior surface of the canon bone, caused by flexion of bone, which is caused by working babies with still soft bone on hard surfaces)
Another under-acknowledged cause of breakdowns in race horses is yearling sales. HUH? I hear you say. Yearling sales cause breakdowns? Indirectly, yes. Horses destined for yearling sales are hot housed and feed lots of calorie dense food, because bidders like fat, shiny, slick yearlings rather than a normal looking yearling - gawky, ribby and scruffy. This contributes to ephiphysitis and OCD (osteochrondosis dessicans, not obssesive compulsive disorder) which leads to lots and lots of joint problems once the horse is in work.