Horse's mouth

My dentist told me that horses have rounded teeth, but they grind their teeth flat as they age. I looked this up to verify and I'm not sure it's entirely accurate, but horses do have something like six different sets of teeth. The point is, you can tell a horse's approximate age by looking in its mouth, which is why you shouldn't look a gift horse in the mouth. Also, I think the later sets of teeth are longer, so old horses are long in the tooth.