Do horses develop KS or is it something that they are intrinsically born with? Could a horse in his teens (regular work, good rider-owner, varied exercise) suddenly develop it after years of no problems?
The surgeon who operated on mine says he sees them at three ages. Three or four when someone first tried to sit on them. Those are born with it. Seven or eight when work starts to cause a problem. Those are either born with it and it's got too sore, or something else has caused it. Twelve to thirteen, when age is catching up with them and I dressage horses where the work is getting too advanced for them to cope.
In a horse of fifteen I would expect it to be something else, but that could include arthritis in the vertebrae which would give the same problems.