[Hwjb]

Well, Brock, I certainly couldn't profess to have more knowledge than you, but I'll have a stab!

First, jumpers are thoroughbreds, descended from the same lines as the stars of the Flat.
Age-wise, there are several things, I think that necessitate their being elder. Clearly speed isn't such a prerequisite over jumps, stamina is, and speed tends to be the first thing to go as a horse matures. On the other hand something they acquire is experience, a kind of canny intelligence. Again, there is no need for this when running over a mile in a straight line or round a couple of bends, but when knowing what to do at a fence, knowing to find a leg on the other side takes experience. Finally, I think, and I'm certainly no expert in this regard, but I would imagine that the immense pressures put on a horse's body would be far more liable to injure or kill if placed on an immature juvenile.
It's worth bearing in mind that around 50% of jump horses come from Flat racing, whereas half are bred by stallions with more stamina in their pedigree and go jumping straight away.

IMO, I would imagine that speed is the overriding reason, and it tends to be over the longer distances of 3m+, not 2m, that horses aged 11-13 still meet with success. Also, for Kauto Star to have reached his level of brilliance at 6 speaks volumes for his future - there are no limits to what he can achieve - and if he wins the Gold Cup in March, at 7, he will do so at the same age as the two three-time Gold Cup winners Best Mate (who won the first of his 3 in 2002) and the immortal, incomporable Arkle (1964-1966).