At what age should a horse's training begin and what is appropriate at each early stage?

The appropriate starting point for a horse's training depends on what type of training is being considered, because different types of work place different physical and mental demands on a young horse. Handling and desensitization work can and should begin from birth or very shortly after, as foals that are handled correctly from their earliest days develop a familiarity with human contact that makes all subsequent training significantly easier. Basic halter training, leading, and accepting routine handling of the feet, ears, and body are all appropriate for foals and yearlings, provided the sessions are kept short and the demands are well within the young horse's physical and mental capacity. Groundwork that develops basic responsiveness to pressure, respect for personal space, and the ability to be led and tied is appropriate for yearlings and two-year-olds as long as it does not involve sustained physical exertion or repetitive concussive work on developing joints. Starting a horse under saddle — asking it to carry a rider's weight — is most commonly done at two or three years of age depending on the breed, the individual horse's physical development, and the discipline. Breeds that mature more slowly, including warmbloods and some stock breeds, are often better served by waiting until three years or later before beginning undersaddle work. The principle that governs all early training decisions is that the work asked of a young horse should never exceed its physical and mental readiness, and advancing the timeline to suit the handler's schedule rather than the horse's development is one of the most common causes of physical breakdown and training problems in young horses.