This section is from the book "The Farmers Ready Reference Or Hand Book Of Diseases Of Horses And Cattle", by S. C. Orr. . Also available from Amazon: The Farmer's Ready Reference;.
As the teeth furnish the best means of determining the age of the horse, a few words upon that subject will not be out of place. The horse has two sets of teeth, the temporary or milk teeth, twenty-four in number, which are shed to give place to the permanent ones, which are forty in number in the horse; the mare, being minus the tusks or canine teeth, has only thirty-six. As it is only with the incisors we have to deal in determining the age of the horse, we will confine our remarks to them alone. If the young foal is born at its proper time, it has at birth, or within a few days after, four temporary incisors, two above and two below: called the central incisors. At about the age of one month four more teeth appear, one on the outside of each of the centrals; these are called the lateral incisors. At about the age of eight months the third set, or corner incisors, make their appearance. At the age of two and one-half years the central incisors are shed and by the time the animal is three years old they will have been replaced by the permanent teeth. The lateral incisors are shed at three and a half and replaced at four; the coiner teeth are shed at four and a half and replaced at five, when, in the male, the canine teeth will have appeared and the animal is said to have a full mouth.
From this time on the age can only be determined by the wear, shape and general appearance of the teeth, to do which requires considerable experience in order to become an expert. As the grinding surfaces of the incisors wear away, their inner walls will come into wear and the cups gradually become more shallow in the same order in which the temporary teeth were shed and replaced by the permanent ones. By observation it will be seen that, as the permanent teeth make their appearance, only the anterior or outer walls come into wear at first, but at the age of six years the posterior or inner walls of the central incisors will be up level and in wear and the cups will have grown more shallow if they have not entirely disappeared. At seven years old the laterals become likewise and at eight the corner ones are the same.
Although the upper teeth wear more slowly than the lower ones, yet they must, to some extent, serve as a guide to the horse's age from this time on; and the same changes will take place in them that the lower ones have already undergone, beginning with the centrals at nine, the laterals at ten and the corners at eleven. The teeth will also have changed their shape to some extent; they will have grown more narrow from side to side and wider from front to back. But these changes will not be the same in all horses; some teeth wear more rapidly than others. On most horses there is a groove on the outside of the upper corner incisor which makes its appearance at the upper part of the tooth nearest the gum at about ten years of age, and, as the tooth continues to rise from the alveolar process, extends down the tooth as follows: At the age of twelve and a half the groove extends down one-fourth the length of the tooth; at fifteen, one-half; at seventeen and a half, three-qarters; and at twenty-one, the entire length of the tooth. While it must be admitted that these marks will be somewhat of a puzzle to the average farmer or horseman, yet with a little careful observation and practice almost any one may be able to determine the age of a horse nearly enough for all practical purposes.