From Wikipedia:

The Galvayne’s groove occurs on the upper corner incisor, producing a vertical line, and is helpful in approximating the age of older horses. It generally first appears at age 10, reaches halfway down the tooth by age 15, and is completely down the tooth at age 20. It then begins to disappear, usually half-way gone by age 25, and completely gone by age 30. The groove is named after horse expert Sydney Frederick Galvayne who claimed he had invented the use the groove to age a horse; however, it had been earlier described by his teacher Professor Hamilton Sample.