Galvayne's groove is a vertical indentation that appears on the upper corner incisor teeth of horses as they age. It first appears at the age of 10 and extends halfway down the tooth at the age of 15. By the age of 20, it extends the full length of the tooth. After the age of 25, the groove begins to disappear. The presence and depth of Galvayne's groove can help determine the age of a horse.