At the age of 1 year:
the horse's pasterns have fused
At 18 months:
his cannon bones are mature
At 30 months (2.5 yrs):
he now has stronger - but not entirely mature - knees (the small bones have fused), and his fetlock joints are mature
At 3 years:
the weight-bearing area at the base of the knees is fused, as well as his hindleg between hock and stifle
the highest part of his foreleg, the humerus, is fused, as are parts of his femur, the area of his hindleg between stifle and hip
At 4 years:
the shoulder is fused, and the hocks and pelvis are now mature
the growth plates over the centrum, which allows the spine to flex, become fused
So most people wait until 4 or 5 to start jumping.
Great post Upsidedown.
Majority of GP Jumpers don't start jumping their horses until 5, and that is very light jumping.
Majority of GP Jumpers, only jump once a week - where most uneducated lower level ridrers, jump more than that. I've even seen GP jumpers only jump their horses in the warmup ring before they go into the show ring.
Fact of the matter is, that jumping is hard on horses joints and ligaments and tendons. A horse only has so much jump, so that is why many Top Level Riders preseve their horses jump for in the competative ring.
And, the longer you wait, and the better you take care of your horse, the longer they'll last. I would definately wait until 5 to start over small fences.