An age old, yet nearly forgotten art, is how to tell a horse's age by its teeth. Simply study the pictures below, memorize the special characteristics of each stage. Then go practice on horses that someone knows the age of. See how close you can come to the real age of the horse. 1  When the horse has grown all its permanent teeth it is said to have a full mouth. 2  The age of the horse can be determined with some accuracy by looking at its teeth. 3  As the horse grows and ages the changes that take place can help you tell how old it is.