How did length of teeth ever get associated with length of life? Gray in the hair? Sure. Stooped in the shoulder? Happens often. But I'll be darned if I've ever seen elongated choppers on someone just because they were getting on in years.

Well, now I've got the answer, and it's straight from the horse's mouth--literally. The expression comes from the world of horse trading, in which the horse's relative age is found by looking in its mouth. Over the years the horse's gums recede from all that chomping on fodder, exposing more and more enamel. Its teeth haven't gotten any longer, but more of their length can be seen. The more you can see, the older it is.

The moral of the story is: If you're a horse, never lie about your age to your dentist.

Source: A BROWSER'S DICTIONARY by John Ciardi)