Hi Squarebear.
The definitive answer is that horse's teeth change quite dramatically with age. As a youngster a horse will only have about half of his full complement of adult teeth. Then a groove known as "Galvyne's Groove" (Think I've spelt that right!!) begins to grow down one side of one of the horse's front teeth. This groove grows down, showing more and more as the horse ages. Then at the age of around 20, the groove disappears as it grows out of the tooth and the horse dentist is left with only the general condition (ie. wear and tear) of the teeth in order to make an estimation of age.
I hope this answers your question.