Horses develop till the age of six. However, they normally attain their maximum height around the age of four or five. From then on, they will not grow anymore. The reason why horses stop growing is because they reach the end of their growth plates. These are sections of bone that grow longer in proportion to each other so that they can support a heavier weight. As soon as a horse reaches puberty, these plates begin to fuse together which prevents them from growing any further. A horse's growth plates close between the ages of three and five regardless of how long it takes it to reach its full size. However, this does not mean that a horse cannot get any bigger. They can, but its bones would become thicker instead. For example, a horse that is being used for work could benefit from having its workload reduced so that it has time to eat more and thus increase its size. Overall, horses stop growing at an average age of five. However, some can stay growing longer than others.