JustDressageit, i know exactly what you mean! A couple of months ago there was this grand prix thing on TV and of course, I told my boyfriend i HAD to watch it! lol Well almost all of the horses were in double digits, maybe there was a 9 year old in there. And you could see the difference between the 9 year old and 12 year old horses. The older ones just seemed to know exactly what to do, and didn't seem to care that they were on TV, in a huge area with hundreds of people around them.
Not to mention if you start jumping a 4 year old 4' right away, do you think by the time its 9, 10, 11 or older that it will be able to jump those grand prix jumps? I'm going to say no.
And I was looking at another horse to buy, to lease to someone to help make some money, well there were like 10 year old horses that could only be lightly ridden, even though it had competed with jumps (i want to say hunter, but i'm having a brain fart)