At age seven, a hook appears on the upper third incisor. This is present in only 60 percent of horses. It reappears at 11 years of age. When aging a horse, it is important to be wary of fraudulent practices. At eight years, stars appear.
Dental stars are the areas of yellow to brown dentin that fill the pulp cavity.
Criteria to differentiate among nine, ten, eleven years rely on the shape of the incisors. Past 11 years, it becomes more difficult to be accurate. At 15, a dark depression known as Galvayne's groove extends halfway down the upper third incisor. Lower incisors become triangular at 16 to 18. At 20, Galvayne's groove extends the entire length of the upper third incisor.