How Old Is the Average Horse When It Stops Growing?

It's hard to give a specific age when horses stop growing, because this varies wildly among different breeds. While most horses reach their full adult height between the ages of 4 and 5, some tall, heavy breeds won't reach their full height until they're around 8 years old. Horses grow much more quickly at first, before greatly slowing down. The majority of breeds reach about 90 percent of their adult height by age 2, taking at least another 2 years to grow that remaining 10 percent.

Horses have growth plates on either end of every bone in their bodies, other than their skull, which allow their bones to grow. These plates at the end of their bones are made from cartilage, but once they've finished growing they convert to bone and become more durable and less susceptible to damage. The plates belonging to bones that are involved in how tall a horse gets -- such as hocks, tibias and femurs -- have usually fused by around 4 years of age. However, the bones of the vertebral column are the last to fuse and rarely do so before a horse is 5 1/2 years old.

As well as physical maturity, you need to think about horses' emotional maturity. Those who are worked too early may have trouble understanding what you're asking of them because they're simply not mentally ready for complex commands yet. Most horses reach emotional emotional maturity between 5 and 7 years old. Training horses too intensively before this time can cause significant problems. For instance, it can make horses become anxious or resentful of their work, which in turn can cause them to exhibit unwanted behaviors to avoid following your commands, such as bolting or rearing.

While you don't have to wait for your horse to stop growing completely and all of his bones to fuse before you get on his back, it's not advisable to start riding him too soon. Although it's rare to cause deformities to the legs or damage the growth plates by riding too young, it can happen. However, it's not uncommon to crush some of the cartilage in the legs. Spinal problems -- such as having a slipped back -- are more likely to occur in a horse who is ridden too young. He may also be unable to work with his back muscles in release. To avoid these problems, it's best not to ride your horse before he's at least 4, but preferably closer to 6.