Experienced horsemen can look at the shape of the teeth, the Galvayne's groove, the angle and the length to make a best guess as to how old the horse may be.
A Colorado State University article says Galvayne's groove would be halfway down the tooth at 15 years old.
Younger horses have wider, shorter teeth and older horses typically have narrow, longer teeth.
Horses who crib will often grasp anything they can causing their front teeth to wear down at a fast rate.
Aging horses by their teeth is a very good skill to learn as a horseman. The more teeth you look at, the better you will get!