Estimating Age by Wear
After the horse is 5, the only way to determine age is by wear, the shape and slope of the incisors and the Galvayne's groove that eventually appears in the upper corner incisors.
In a young horse there are cups (indentations) in the center of the tooth's grinding surface. The cups are usually darker than the rest of the surface, and a ring of enamel surrounds the cup. As the tooth wears, the cups gradually disappear. In the lower incisors, the cups disappear from the centrals at about age 6 and from the corner incisors at 8.
Cups in upper incisors disappear a bit later (from the centrals by 9, the next ones by 10 and the corners by age 11). The horse is then smooth-mouthed. These changes are gradual and not exactly the same for each horse, since wear can vary with diet. Horses biting off grass wear teeth faster than horses eating hay, grain and pellets.
Other Indicators
Up-and-down ridges on the outside of the incisors start forming on the centrals at 10 years, then on the middle teeth and last on the corners. The corner incisors have ridges by the time the horse is 14 to 16 years old.
Teeth also change shape as the horse gets older. At first the permanent teeth have oval surfaces. By age 12, the surface of the central incisors has become round. By age 17, all of the incisors have round surfaces. By age 18, the centrals are more triangular than round, and by age 23, all the incisors are triangular. After that the grinding surfaces again become oval.
It's easy to tell the difference between the oval-surfaced tooth of a 7-year-old and the oval tooth of a 25-year-old. The young horse's teeth are broad across the front and narrower from front to back. The old horse's teeth are long from front to back and narrow from side to side. Young teeth are also shorter and straighter when viewed from the side. They meet at almost a right angle. Older teeth are longer and protrude forward.
The older the horse, the more his incisors slant forward, coming together in a point like the beak of a bird. Teeth are quite long by the late teens, but may become shorter again in very old age as they wear away.
Upper-corner incisors develop hooks on the back outside surface, which change with age. A hook usually starts to appear at age 8 or 9 and is fairly pronounced by age 12. If there's a hook on the corner tooth, the horse is older than 7. By the teens, this hook starts to wear off, due to changing angle of the incisors and different wear points. By 16, the hook is usually gone.
The gum line on the inside of the mouth also changes with age. In a young horse, it's fairly straight across the tooth. In an older horse, the gum sags down, making a more irregular line (a scallop effect) and exposing more of the middle shaft of the tooth. Another clue to age is that the inside of the jaw becomes thinner as the horse gets old.
Normal Variations
The wear and length of incisors can vary, which is why it can take some skill to learn to read teeth. Teeth of different individuals may age at different rates, depending on genetics. Some horses have old, slanting-forward teeth at earlier age, sometimes depending upon nutrition and environment.
A horse fed hay and grain all his life (rather than biting off grass) will show less wear on his front teeth and they will also be longer. A horse pastured on sandy soil may wear teeth faster due to the abrasiveness of chewing sand with his feed. (A 6-year-old eating short grass in sandy soil may show an 8-year-old mouth.)
A horse with an overbite or underbite (parrot mouth, sow mouth) where front teeth don't meet properly won't have much wear, and the unopposed tooth will grow too long. A horse that chews wood or cribs will wear down his incisors more rapidly than normal. Dental work to smooth up an uneven mouth may also change the appearance of teeth, making it harder to tell the age.
