What are the physical development milestones trainers should know about before starting a young horse?

Understanding the physical development timeline of young horses is foundational to making responsible decisions about when and how hard to push training, and the basic knowledge of growth plate closure and skeletal development is something every trainer and horse owner should have. Growth plates — the cartilaginous areas at the ends of bones where growth occurs — close at different times in different parts of the body. The growth plates in the lower legs (pasterns, fetlocks, and cannon bones) close relatively early, typically by eighteen to twenty-four months. The growth plates higher in the body — the knees, hocks, and especially the spine — close significantly later, with the spine and back being among the last to fully mature, often not until the horse is five or six years old in many breeds. This developmental timeline has direct implications for training. Work that is concussive, repetitive, or that places significant stress on the spine — including carrying a rider's weight — before these structures are mature carries genuine risk of damage that may not be immediately visible but that contributes to soundness problems later in the horse's career. Warwick Schiller has addressed this directly in his teaching, noting that the industry standard of starting horses at two is commercially motivated rather than biomechanically optimal, and that horses started lighter, later, and more progressively tend to have longer, sounder careers. This does not mean horses cannot be started at two — the industry does it routinely and most horses handle it without incident when the work is appropriate to their development stage. It means that the two-year-old under saddle should be doing walk and trot work on good footing in short sessions, not repetitive drilling, hard circles on tight footing, or demanding physical work that stresses structures that are not yet mature.