I have heard that you can tell how old a horse is by looking at their teeth. Does anyone know how this can be done?

A horse has two sets of teeth in its lifetime. The first set is the foal teeth- obviously foals have them. The permemant teeth that replace them continue to grow throughout the horse's life, but will vary is shape and distribution of enamel as they are worn down.

Basically if a horse's teeth are flatter, it's younger, and if they're rounder, its older. There are heaps of charts online yuo can find that will let you compare how the teeth change over time.