Age does matter. At the age of your horse, he should be around 95% of his mature height. There are two ways to do this:
(1) First, measure your horse's current height. Divide the present height by 95% and then multiply by 100.
(2) Second, if you measure from the knee to the ground, you will divide that measurement by 95%, multiply by 100, and again multiply by 4 to give you the height.
A third method is to take a tape measure that is marked in centimeters. The horse will need to be bare-footed or recently trimmed or you need to allow for shoe height. With the horse standing perfectly straight and square, put the tape on the point of the hock and measure the length to the ground. This measurement in centimeters will translate to inches at the withers at maturity. For example, a horse with a hock-to-ground measurement of 62 centimeters will be around 62 inches in height at the withers at maturity.
Here is a method you might use in guesstimating a horse's final height. Take a string or small rope. Start at the elbow and run the string down to the fetlock. Tie a knot at that point. Then again start the string at the elbow and go upward over the withers. Have someone measure the height with a measuring stick or a tape measure while you are holding the string in place. This will give you an estimate of the horse's mature height. Again, this is a good guess and should be within an inch or two.
