My horse is 19 and he hasn't changed one bit since I bought him. He loves to work and gets very grumpy and mean when he isn't worked. I think you really just need to judge on how she is acting. A competive trail horse that used to live around here was 38 when he passed away and he was still competing. The vet that knew him said the only reason he was put down was because it was too cold for him to be outside and he layed down and couldn't get up. I still show my guy and he loves it, my vet said to keep going as long as he is happy.