Depends whether you're talking morale values or purpose- TOP competitors in the jumper world and dressage are started in consistant work as young as 2, and jumpers by 3 are going around doing courses. Barrel racers and western sport horses are started that young as well. Different horses mature at different speeds and rates and it's up to you to decide. Please, I don't mind this debate giong on, but please just not on my thread? :) Thank you!
Just had to add my opinion too:)
Yet again you show your ignorance. Horses all Grow at exactly the same rate, sure some look more mature earlier but thier growth plates all seal at approximately the same time and in the same order (irrigardles of breed).
Top Dressage riders do not start them at 2, Most won't even think about acking a horse untill 3 or 4 years old. They cannot be competed untill 4 years old and generaly cannot mentaly cope with the work required of them when younger.
Showjumping it used to be that you could not compete untill they were 5 years old, I believe that has now been reduced to 4 years old. Top showjumpers over here do not break thiers untill 3 and a half and then only have a light season as a 4 yearold. I believe the the famous "milton" didnt start jumping untill he was 4, didnt get to international level before he was 8. Top show jumpers prefer to preserve thier horses and keep them fresh!
Starting a horse too early not only damages hocks and increases the likely hood of early artheritis but is can mentaly destroy a horse. You push a horse to young and you end up with either a dangerous horse or a horse who will go sour well before it ever reaches its potential.
For every year you give a horse at the start of its career you will get 3 back at the end on it's career.
I'm not even going to start on my thoughts about how young some western horses are ridden!