Like any other breed of horse, mustangs come in all shapes, sizes, and personalities. However, one big difference between most domestic horses and mustangs is how they grow into those sizes!
The average Quarter Horse (and most domestic horses) reach maturity at 4 or 5 years old; the size they are by then is generally the size they will stay for the majority of their lives.
Mustangs, on the other hand, are known to grow until they are 6 to 10 years old! I remember one particular Mustang mare that had come through the Trainer Incentive Program (TIP), who arrived for training at 7 years old and was roughly 14 hands. I saw her again 7 or 8 months later, and she was nearly 15 hands! Talk about a growth spurt!
Unlike her, however, most mustangs' growth rates slow as they get older. Like any normal horse, most physical changes occur when they are young, but then continue to steadily mature until they are fully grown--at whatever age that might be!
This raises another question: If they grow for so long, how big will they be when they're finally matured?
It would be funny to think that Mustangs grow up to be 15 hands at 5 years old, and then just keep getting bigger! But actually, Mustangs are of a very average height, if not even considered to be short.
Most Mustangs rarely exceed 15 hands; the majority of Mustangs measure between pony-sized to 14 hands. Of course, as with any breed, there are always exceptions! Our local holding facility once had a gelding that measured 18 hands!!
More information concerning the Trainer Incentive Program is available here: http://www.mustangheritagefoundation.org/tip.php
