You suggest steeplechase blood? Instead, check out the average age of a steeplechase horse.
If you have a horse you want to keep for a dozen years, you start with only the lightest exercise at two years old. You gradually increase the demands--you don't want to injure those bones and connective tissue if you want to still have your friend around after ten years. But these high-strung T-breds? Run 'em till they drop, sell the geldings and hope the mares will last long enough to breed.
"A three-yr-old horse is too young to run with weight on his back for these distances..."
No offense, but 132 years of Kentucky Derbys says they can.