I think it may have something to do with telling a horses age from its teeth.
Horses can be passed off as almost any age between the ages of 3ish and 12ish. The only sure way is to look at the teeth. From the age of 3 to 7a mark known as Galvanes Groove begins to run down the tooth However after the ages of between 7 and 9 the mark begins to disappear and 'old' teeth can start to look like young teeth again. Obviously only until the horse is really old and its teeth fall out.
Having said all that it probably nothing to do with at all. I'll go now.