Around 3-5 years, duck fertility and egg production lessen, and many keepers choose not to keep older birds because of this. But there are many reasons to keep ducks into their older age. We discuss some of them here. Other reasons to keep older birds can be found in this creative My Pet Chicken blog article (it refers to chickens but the same ideas apply to waterfowl): Older hens, Kenny Rogers, and long-term flock management.
Geese like the White Chinese normally live 10-15 years.
Unfortunately, since ducks and geese are prey animals, the main cause of death is usually predators--especially dogs. So the most important thing you can do to help your waterfowl live a long, happy life is to provide predator protection for them.