Long in the tooth

I used this expression today, and stopped practically mid-sentence to ponder-- where does it come from, originally?  These days, it's used to refer to almost anything or everything, regardless of how long its tooth is, or even if it has teeth to begin with (I was actually referring to some positively toothless leftover stir-fry).

I forthwith learned several things, to wit: firstly, that the expression was originally used to refer to horses.  This makes a certain amount of sense.  After all, the use of horses in phraseology is well established, from horses that are gifts to ones that are of different colors to ones that you merely wish to make drink.  They are apparently richly symbolic animals.

Secondly, I attempted to track down the reasoning behind the saying, and this is where it gets a bit more complicated.  Supposedly, at least according to the Encyclopedia Britannica (and some other random online sources), horses (somewhat like rodents and exceedingly unlike humans) have teeth that continue to grow throughout their lives.  In the wild, the act of grazing provides more or less enough wear to keep these teeth in check for most of a horse's life, but the domesticated diet is softer and the teeth of domesticated horses continue to erupt and lengthen.  Thus, the length of a (domestic) horse's tooth is roughly correlated to their age.  However, the Oxford English Dictionary, which is generally fairly reliable, defines the phrase as "long in the tooth: (orig. of horses) displaying the roots of the teeth owing to the recession of the gums with increasing age; hence gen., old." which, while similar, is not quite the same thing.  I don't know much about horses myself-- I tried riding one once, and neither of us enjoyed it very much.  If God had meant us to ride horses, he wouldn't have given us bicycles.  Anyway, I digress.

So we are actually left with a bit of a mystery, with two slightly disparate explanations of a phrase that (regardless) definitely came straight from the horse's mouth.