I wondered out loud about this on Monty's journal, but I thought I'd pose the direct question here.
First let me clarify by saying, I have some ability to estimate a horse's age by teeth, but not a great amount.
I got Monty, my rescue hackney, being told he was a three year old.
He's finally gotten where he will let me open his mouth and have a look, and it's set me wondering if I was told wrong. He is supposedly registered, but I have not been sent his papers yet, and being as the previous owners got him from Shipsy auction, there is a good chance papers are fake anyways. They only had him a short time, and couldn't even touch him during that period, so while he was seen by a vet, I doubt anyone looked in his mouth.
His teeth are ALL baby teeth, to the best I can tell. He also has what I thought might be an old injury to his jawbone, but what I am now suspecting is actually his permanent molars that have not yet erupted - a row of regularly spaced bumps along his lower mandibles, same on both sides, I'll try to put a picture below. I don't feel the same bumps on Finn or Fiona, but they are a good bit fleshier and heavy boned than little refined Monty.
(not a very good picture, but you can see the thicker spots on his lower jaw. They are hard, calcified feeling, not soft like abscesses etc, and not sore to handle at all. I'll try to get an actual picture of his teeth tomorrow - no promises though! He's coming along, but still has his moments!)
So, to the best of my understanding, having no permanent teeth yet puts him at only two or two and a half years at the latest, is that right?
I got to wondering about it due to the fact he has shot up an inch in height since I got him. I know it's not uncommon for particularly younger rescues to start putting on some height once they are getting enough to eat, but he has also started getting a bit of that butt-high, baby appearance I generally associate with gawky, mid-growth-spurt yearlings. I'll try to get my vet out soon for a best-can-tell accurate age estimation.
I feel really bad for having been riding him around now. :(