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.
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.