When I say young, I mean horses younger than five years of age.
This is my opinion:
I think that, regardless the height of the jump, horses five years and older can jump whatever height.
I think ground poles is fine. Espically streching over them, etc. It gets them somewhat prepared. Espcially, ground poles that are painted. It'll help them get a great sense of "jumps are okay, no matter the color."
The reason behind this is because their joints don't completely close until five. That's why I'm against racing, because it puts so much strain on the horse.
That video makes me really mad, because that horse is only one. I know the jump is only 6inches (or whatever) tall, but he's still a baby. I know he seems to somewhat enjoy it, but still, I don't think his joints do!
So share your opinions. This is a debate/discussion, but don't take it out of hand.