Most horses are not done completely growing until they are about 6 or 7 years old. Most of the noticeable growth is done between birth and about 3 or 4 years, with the obvious butt to withers growth spurts happening. Warmbloods and draft horses in particular are very slow growing horses, with some even at 8 still growing. But just as with people, not all horses are the same. I have an Arabian that at 4 years old is still doing large growth spurts, and will continue to grow until she's about 7, based on her bloodlines, but I have also known horses that at 3 or 4 are just about as tall as they are going to be, and the rest of the growth is just bulking up. The easiest way to check, is to stick the horse (measure), and a month later, stick him again, and then the month after that, ect., and see what the results are.