How to Tell a Horse's Age by Its Teeth

The eruption times of the upper central incisor (2.5 years), upper middle incisor (3.5 years) and upper corner incisor (4.5 years) for horses under 5 years of age is considered fairly accurate in determining a horses age

The new permanent teeth are quite concave on their surfaces and these "cups" along with the angle, shape and a groove on the outer vertical surface of the upper corner incisor (UCI) that gradually grows out, called Galvayne's groove, are the indicators of how old a horse might be.

A "hook" located on the rear end of the UCI is often visible at age 6-7 years, and then again at age 11-13 years. Due to this variability in occurrence, it is not considered definitive in determining a horses age. 2

As the horse wears its teeth by grazing, the concave surfaces are worn flatter. By the time a horse is about 11 years old, it will have worn its teeth flat. How quickly this happens depends on the type of grazing it has available. Horses that graze grass on the sandy ground will wear their teeth flatter than those fed only hay grown on clay type soil. 

At about age 10, the Galvayne's groove begins to appear at the gum line. This groove will continue to grow out so that reaches the entire length of the tooth. By about age 15, the groove will reach half-way down the tooth, and it will reach all the way down the tooth by age 19-20 years. By the time a horse is in its mid-20s the Galvayne's groove begins to disappear from the gum line, and if the horse lives long enough, will disappear completely as the tooth naturally wears away.

Senior Horse Teeth

As the horse ages, the angle of the teeth increases. The milk teeth are short and straight as are the newly erupted permanent teeth but as the horse ages, the teeth become more angled and the UCI becomes more tall than wide. This is where the saying "long in the tooth" comes from, because as the angle increases, so does that length from the gum line to chewing surface. The teeth also change in shape from oval to more angular as the horse ages. The color of the teeth will probably be quite yellowed and stained.