Horses, then, ought not to be raced until they are five. But since younger horses have faster times, Is it that older horses just don't like to run?
IAC, I am reminded of Little League pitchers who have thrown out their arms by the time they are fourteen, and young quarterbacks the same age who have bunged up knees. That happens a lot. A physical trainer I once knew told me that no youngster should have to do really stressful sports until he/she has completed 90% of his/.her growth. Yet such are the demands of sports mad parenents/coaches that fast developing kids are pushed to the limit and because of their success, then push themselves beyond them.
I didn't see a reply to your post, so I thought I'd chime in. In theory you are correct about racing older horses. Unfortunately this is a high dollar sport and so the owners, trainers, breeders, etc are looking for a turnaround on their investment ASAP. I only occasionally watch racing now because of how the human greed factor pushes these young athletes.
I own arabian horses, which are more slow to physically mature and do seem to live longer on the average. My last youngster was put under saddle at age 2 years 6 months. He was too mentally and physically active and I was afraid one of us was going to kill the other in frustration. All he learned to do at that point was walk, trot and canter carrying a rider. At that same age most thoroughbreds been under saddle for a year. They're hard at racing and only the strong survive.