When should I start a young horse under saddle?

The question of when to start a young horse is one of the most debated in horsemanship, and the honest answer is that it depends on the individual horse's physical and mental maturity more than its age on paper. The growth plates in a horse's skeleton close at different times in different parts of the body, with the vertebrae and the spine being among the last to fully mature — often not until the horse is four or five years old. This does not mean a horse cannot be started under saddle before then, but it does mean that heavy work, collected work, and repetitive concussive exercise on hard ground carry more risk in a younger horse whose skeletal system has not finished developing. The industry norm in many western performance disciplines is to begin light riding work at two years old, with more demanding training increasing through the three and four-year-old years. Thoroughbred racing begins earlier. Warmbloods destined for upper-level dressage or jumping are often started later and brought along more slowly. The most useful guide is the individual horse in front of you: a two-year-old that is mentally calm, physically balanced, and well-muscled through the topline is a better candidate for beginning work than a three-year-old that is still gangly, unbalanced, and mentally immature. Starting a horse late is rarely a mistake. Starting one too early and breaking it down physically or creating lasting anxiety is a mistake that is difficult to undo.