The average breed’s knees close at 2 years of age. Most breeds will achieve maximum height by the age of 5 while some can grow for up to 7 years. Young ponies have a complex bone structure with cartilage tissue to protect each end. As the bone structure grows stronger a bond will be created. This bond within the knees is known as knee closure and trainers usually check for this before starting mounting upon the yearling. However, always consult an expert before you start riding upon a younger horse.