The average thoroughbred racehorse weighs around 1036 to 1135 pounds. The average weight of a mature thoroughbred is 1256 pounds. These horses are raced at very young ages, usually beginning at around 2 years of age. Thoroughbreds are not physically mature until 5 years of age, at which point most have been retired from racing.
The thoroughbred has been selectively bred to grow quickly and reach its mature size very early. However, its skeleton does not finish maturing until age five. Thoroughbreds suffer from a very high rate of breakdowns and injuries and it is unclear if selective breeding for extreme speeds is the cause, or if the practice of racing immature horses is the cause. It is possible that the practice of sending injured racehorses to the breeding farm has resulted in selection for horses that tend to break down.
Immature breeds of horses used in other equestrian sports such as reining also suffer from high rates of injuries and breakdowns. Horse sports that wait until the horses maturity (such as dressage) before working them tend to have much lower rates of injury and breakdown.
The rationale for racing thoroughbreds at such young ages appears to be entirely financial.