You stated the following: "Horses are not even physically mature until 5 or 6. They can then maintain their physical condition for several more years."

The authors of the study I referenced stated the following, when summarizing their findings: "We find that a typical horse’s peak racing age is 4.45 years."

Further, their study (Table 4) found that the average 4 1/2 year old racehorse runs 5 points faster (Beyer scale) than a 6 1/2 year old racehorse. As such, contrary to your claims, racehorses have already peaked and are generally running slower speed figures by 6 years of age.

Therefore, the conclusions of their study do not support your statements.