As far as teeth go, I wasnt referring to later years but a vet could tell you if they are younger or older. This can be done with most animals not just horses. I had it done with 2 chins and two cats as well so yes it can be done. It's easier with other animals who lose teeth like humans do ( ie cats and dogs). No it's definitely not accurate especially after a point because of course yes molars do keep growing.
I dont know the science, however as im sure you have noticed baby chin teeth do look different from a 10 year old chins teeth. There's color, condition, calcium build up and such.
As i've said though it would only be a range/ball park age though