Age appropriate training: when do horses physically mature

There has always been great debate among equestrians as to what is considered the best and most appropriate age for training a horse specifically in relation to when to begin riding.
Traditional wisdom is that a horse should begin his or her ridden career at three years of age for other riders it is to wait a few years more whereas in areas such as the racing industry horses are commonly under saddle and on the track by two years of age. But do any of these opinions take the physiology of equine development into account?

When do horses physically mature?
No horse is skeletally mature until around six years of age at the earliest with some not reaching full skeletal maturity until eight years of age. This is not breed specific despite what is sometimes believed.
Factors that affect the growth of both the skeleton and structural soft tissue such as the ligaments muscles and tendons can include genetics nutrition diet gender environment and stress.
Just as malnutrition can result in stunted growth overfeeding highenergy grain to the young horse has been identified as a key aspect in the development of DOD Developmental Orthopedic Disease caused by an accelerated growth rate.
How do horses grow?
Knowing how a horse develops skeletally we can make an informed decision about what is best for our horses.
Skeletal development refers to the formation of bone this begins in utero and does not stop until the horse has fully matured.
A key part of how any skeleton forms is down to the growth plates. Growth plates are found in all mammals they are areas of cartilage that lengthen the bone and are found at the end of every bone behind the skull.
In the case of bones with multiple angles like the pelvis there are multiple plates in these areas. Susceptible to injury and