I certainly won't pretend to have the answer to that question. Like people, each horse is an individual, subject to their own rate of aging, effects of past injuries, illness, and infirmities. Some breeds, like Arabians, are known for their longevity. I've heard, from reputable sources, that one of the keys to their longevity is continued use and exercise.
Smokey turned 21, this year. I find myself more acutely attuned to his health and well being. Towards the end of summer, I found myself wondering if, perhaps, he was getting too old for what I ask of him.
The other night, as I was bringing him in from the pasture, I couldn't help but notice that his eyes are bright, and he has a spring in his step. And, in the arena, the old boy was hotter than a two dollar pistol. He wanted to go fast. Being an equine speed junky, I of course, let him.
Nope, he's not too old.