A horse's teeth can be used to determine a horse's age up to about nine years. After that, age can only be estimated.
Young horses have temporary milk teeth which are smaller, whiter and smoother than permanent teeth.
By the time a horse is one year old, it has a complete set of 24 milk teeth. At two, the teeth are more fully erupted and the upper and lower sets of teeth are touching which causes wear. At age 10, a groove develops on the gum line. This is called Galvayne's groove and as the horse ages, it extends downward. By age 20, the groove is all the way down the surface of the tooth.