Which physical feature is commonly used to determine a horse's age?

The physical feature commonly used to determine a horse's age is the teeth. As horses age, their teeth undergo various changes in shape, wear, and eruption patterns that can provide significant clues about their age. For example, horses have a set of temporary or "baby" teeth that are replaced by permanent teeth as they mature. Furthermore, the presence of galvayne's groove—a mark that appears on the upper incisors—can provide insight into a horse's age as it develops and recedes over the years.

While other features like tail length, height at the withers, and weight can provide some information about a horse's condition or breed standards, they are not as reliable or consistent for age estimation as a horse's teeth.