Unfortunately, as with so many things with horses, the short answer is “it depends”.
Usually a Miniature Horse has reached most of it’s adult height by the age of 3 or 4, but depending on the individual, it could easily be as early as 2 and as late as 6 before they reach their full height. And if the question is one of maturity, then the answer is always 6 – that’s how long it takes for their bones to mature and growth plates to close. But since Miniature Horses are a height breed, it’s how tall they are that most people are concerned about.
It can be helpful to ask the breeder of your horse when their horses usually reach their mature height, as it does seem to be related to bloodlines – if their siblings tended to be done growing early, for example, probably they will be as well.
Another variable is whether they are gelded or not. A stallion generally (again, individuals vary) reaches their mature height around the age of three, while a gelding can grow until the age of six, due to the role of testosterone in closing the growth plates.
All horses do the bulk of their growing in their first year of life. In our experience with our herd, they usually gain approximately 2 inches after they’re a year old, but that may not hold true for all horses or all herds.
Many years ago my Grandad bought a mare who was showing at the AMHA National Show in the 32” and under senior mare class. She was already a mature mare, 4 years old. And after we got her home she grew to 36” tall. Even allowing for variations in measurement (because, despite what we might wish, measuring is not an exact science) she grew a TON as a mature mare. Of course, this is not the case usually, but it does help illustrate the fact that they CAN grow until they’re six years of age.
But I’ve also known AMHA registered horses who were too tall to show under the height limit as yearlings (32”) and two year olds (33”) who never went over 34” and weren’t even super close to the limit at adults. They just did all their growing early, and then stopped.
Their nutrition can also have an effect on growth rates – if they haven’t had an appropriate diet in their first year they may seem very small, but then catch up to their genetic potential height in a hurry when they get some good nutrition.
So – your Miniature Horse will probably reach their full height somewhere between 2 and 6 years of age depending on their genetics, nutrition and hormones, with an average of 3-4 years. Ish. 😉
And no matter what height they mature, they’re perfect just the way they are!
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