Long in the tooth
Old, especially of horses or people.
Horses's teeth, unlike humans', continue to grow with age. They also wear down with use, but the changes in the characteristics of the teeth over time make it possible to make a rough estimate of a horse's age by examining them.
There are various similar Latin phrases dating back to the 16th century. The gap between these and the first citation of the English version - in 1852, make it likely that 'long in the tooth' was coined independently from those earlier Latin sayings. That earliest citation is in Thackeray's, The History of Henry Esmond, Esq. and refers to a woman rather than a horse:
"His cousin was now of more than middle age, and had nobody's word but her own for the beauty which she said she once possessed. She was lean, and yellow, and long in the tooth; all the red and white in all the toy-shops in London could not make a beauty of her."