What a remarkable turnaround from the photos. This mare is so lucky to have you!! I hope they shut that riding school down too...its just awful what some school horses are put through at certain facilities.
I think she's closer to 13 than 23 just looking at the later photos, but I too would suggest having a vet out to look at the teeth just for a more accurate guess
Hey to your question of age I would think 13 because 23 is old for a TB they really age badly and you can tell. She looks fantastic to me. Also if she is branded you should be able to find out on the thougherbred registry, even if you don't have papers.