Examining a horse’s teeth to estimate its age is one of the oldest diagnostic skills in equine practice. It works because horses’ teeth erupt on a predictable schedule, wear at a predictable rate, and change shape and angle in ways that correspond to age. The estimate is useful and often close, but it is not a birth certificate: beyond roughly ten years, error margins widen, and individual variation, diet, and dental work all affect the picture.
Tooth Types and the Aging Surface
Horses have two sets of teeth: deciduous (baby) incisors that erupt in foalhood and permanent incisors that replace them, plus canines, premolars, and molars. Aging by teeth focuses almost entirely on the six upper and six lower incisors, which are visible without a speculum and wear in a consistent pattern throughout life.
Each incisor has a cup (an infundibulum, a pit of enamel in the center of the biting surface). As the tooth wears, the cup shallows and eventually disappears. Below the cup remnant, a darker oval called the dental star appears; it is the pulp cavity progressively exposed by wear. The shape of the biting surface changes from oval in youth to round in middle age to triangular in old age. These transitions, along with the angle the teeth make when viewed from the side, are the primary aging indicators.
Deciduous Eruption (Birth to 2.5 Years)
Foals are born with their central incisors already erupted or erupting within days. The intermediate incisors (the pair on either side of the centrals) follow at roughly six to eight weeks. The corner incisors (the outermost pair) come in at six to nine months. All three pairs are in wear, meaning they contact the opposing teeth and have begun to show a cup, by roughly nine months of age.
A foal with all six deciduous incisors in wear, showing small cups, is approximately nine months to one year old. Deciduous teeth are smaller and whiter than permanent teeth, with a distinct constriction at the gumline called a neck, a feature absent on permanent teeth. This constriction alone reliably distinguishes a deciduous tooth from a permanent one at a glance.
Permanent Eruption (2.5 to 5 Years)
The permanent centrals erupt at approximately 2.5 years and are in wear by three years. The permanent intermediates erupt at 3.5 years and are in wear by four. The permanent corners erupt at 4.5 years and are in wear by five. A five-year-old horse is said to have a “full mouth”: all permanent incisors erupted and in wear.
During this window, the mouth is a mix of large permanent teeth and smaller deciduous ones, which makes the age determination reliable and precise. A horse with permanent centrals in wear, deciduous intermediates still present, and no corner permanent yet visible is very close to four years old. These transitions are narrow enough that an experienced examiner can estimate age to within a few months in a young horse.
Canine teeth, present in most males and some mares, erupt at four to five years. Their presence confirms a horse has reached at least four years of age.
Wear Patterns from 5 to 15 Years
Once the full permanent dentition is in wear, aging shifts from eruption sequencing to wear pattern. The cups on the lower incisors disappear in the same order they appeared: the central cups are gone by roughly six years, the intermediate cups by seven, and the corner cups by eight. This transition (“smooth mouth” in the lower incisors) indicates eight years old or older.
The dental star appears first as a narrow yellow-brown line just in front of the cup remnant on each tooth. By ten years, the star is a rounded dark spot near the center of the biting surface. By twelve to fourteen, the cup is gone from the upper incisors as well, and the star occupies a central position on both arches.
The shape of the biting surface is read from the lower corners. At five to six years, the corner incisor biting surface is wider than it is deep: an oval with the long axis running lip to lip. By nine to ten years, it approaches a circle. From about thirteen to fifteen years, it becomes noticeably longer front to back than side to side, beginning the triangular profile that characterizes the older horse.
Galvayne’s Groove
Galvayne’s groove is a longitudinal groove on the outer (labial) surface of the upper corner incisors. It appears at the gumline at approximately ten years, has extended halfway down the tooth by fifteen, reaches the bottom of the tooth at twenty, recedes back toward the gumline by twenty-five, and has disappeared entirely by thirty.
The groove provides a useful marker in the fifteen-to-thirty range, where other indicators become less precise. A groove that has reached the midpoint of the tooth places the horse near fifteen years. A groove extending the full length of the tooth suggests twenty. A groove that has begun to recede from the lower end suggests twenty-five. These are approximations; the groove’s rate of travel varies between individuals and between the left and right corners of the same horse.
Angle and Profile Changes
Viewed from the side, the angle that the upper and lower incisors make against each other changes with age. Young horses have nearly vertical incisors meeting at close to ninety degrees. As the horse ages, the teeth take on a progressively more slanted, forward-projecting profile. By fifteen to twenty years, the incisors project noticeably forward, and by twenty-five or older the profile is markedly oblique. This change is caused by the geometry of a long tooth erupting slowly over decades combined with the gradual forward migration of the teeth.
The seven-year hook (a notch that forms on the back edge of the upper corner incisors at approximately seven years and again at thirteen) is cited frequently but is an unreliable individual marker. It appears in many horses near seven years and disappears by eight, but it is absent in a significant proportion of horses and its timing is inconsistent enough that it should be used as a supporting observation, not a primary indicator.
Accuracy and Its Limits
The methods above are more accurate in young horses and less accurate in old ones. Before five years, eruption timing is reliable enough to estimate age within months. From five to ten, a careful examiner using cup disappearance, dental star position, and biting-surface shape can usually estimate to within a year or two. From ten to twenty, Galvayne’s groove and angle changes narrow the range, but the estimate is typically plus or minus three years. Beyond twenty, dentition aging becomes a rough bracket rather than a point estimate.
Diet affects wear rate: horses on abrasive feeds or sandy pastures wear their teeth faster than horses on hay or soft pasture. Dental floating (filing the teeth to remove sharp edges) removes some surface structure and can obscure cup patterns. Inbreeding and breed differences affect eruption timing modestly but measurably: draft breeds tend to erupt later than ponies. A horse that has been floated aggressively may appear older by cup criteria than its actual age.
For any commercial transaction or clinical decision that turns on precise age, a veterinary examination combined with available records is more reliable than dentition alone. The teeth give a useful estimate; they do not give a date.