Breed and size can modify a horse's lifespan. Say we're talking about horses that are healthy and live in a fairly stress-free, happy environment. Many large horses that have been bred for athleticism, attitude, and looks alone don't tend to live much past 20, maybe 25. This mostly includes Clydesdales, Warmbloods and Thoroughbreds. Draft horses (the more hardy ones like Shires and Percherons) are usually closer to the 25 mark. Clydesdales are draft horses, yes, but they tend to be more lightly built and are mostly bred for showing rather than hard work. Smaller sport and riding horses (the largest group) like Quarter Horses, Tennessee Walkers, and Andalusians usually live to be at least 25, sometimes 30. Smaller breeds like ponies, Appaloosas, Mustangs, and Arabians can (and often do) live well past 30. I think I once read about a Shetland pony mare that had just celebrated her 52nd birthday!