A horse's growth in height is essentially complete by the age of 3, but his spine takes a full five years to mature. This means your horse will continue to grow in length after he stops gaining in height. It's also a good reason to heed the cautions to take it easy with work under saddle at an early age. A postmortem study performed at the University of California, Davis, found some type of bony spinal disease is 97% of Thoroughbred racehorses. Racing Thoroughbreds start training at an early age, as do many young western performance horses who are slated for 2- and 3-year-old futurities.