I entirely agree with Sara! It really depends of the horse, its health, its daily work, its moral.
I honestly think it depends entirely on the horse. My mom adopted a Thoroughbred who was officially retired from racing at 6. My show horse retired at 15 when he began to develop navicular. Then, there was a horse at my college who was 26 and still kicking. He could out-jump every other horse on the property & didn't show any signs of wanting to slow down. I think it all depends on how strenuous the horse's work load is, how well the horse is taken care of, and whether or not they develop any major health issues.
I like to retire a horse at 15 or 16. Reining solicits a lot a horse's limbs.
In eventing, it really depends of the horse's health. I plan to retire my horse at 18 years old, even he does not seem to be tired, I prefer as the body (heart, tendons, articulations...) is more delicate and eventing is a quite trying trial for horses.
In my club, we retire horses at 18-19 years old, or before if they are tired or in bad health. I guess it really depends of the horse's state and health!
It mostly depends of his condition. When I will see he's getting tired and bored, I will let him in peace. For the moment he is in shape! :)