Just some thoughts, no scientifically researched information, nor degrees opinion.
Bodies adept to stress, both young and old.
Use creates strength in both young and old.
Human athletes who begin as tiny tots generally out perform those who come to the sport as a teenager or adult. I am thinking particularly of gymnastics and swimming and ice skating, but believe this carries through other sports as well.
Lack of use creates weakness. (Use it or loose it is so true)
The horse needs to learn so much, to gain so much strength to do what we want.
In nature he is expected to keep up with the herd from almost the moment of birth. He has the speed to do so. In nature there is no sparing the baby nor the youth, and just maybe the horse is built to take a degree of hard use as a youth.
Again, this is just thinking out loud, thoughts to wonder about.
Could it be possible that the soft plates prevent breakage, and can repair and strengthen because they are still growing? Maybe during youth is the best time to improve strength, agility, recoveries, sure footedness?
In an adult horse we condition only to the point before injury, but to where the body will rebuild it's self stronger. Youth is the very special time for growth and building. The body is programed to do that. When adult, some of that very specially ability to adapt to demand, to the conditions of the environment, is lost.
Perhaps programing the body in youth in the direction of the adult requirements will create a stronger, more specialized body.
Maybe the rule to remember is not to condition to the point of injury, but only to the point where repair will build a stronger system, be it respiratory, bone, mussel or mind.
Horses break down from use. Horses get stronger from use. Both statements are true. I am beginning to wonder if that might be possible during both youth and as an adult. The wise choice might be to be careful not to cross the line of over use at any age.
Perhaps in some racing stables some limited stress is put on youth to find the individual who is not fast enough or structurally strong enough to go into the program, and also to stress the body just enough to build it stronger and faster.