It is possible that most wizards/witches die not of old age, but simply because being a wizard is such a HUGE occupational hazard. If you use magic every day, you can make a fatal mistake eventually; especially if your magical power keeps increasing with age, whereas your memory / ability to concentrate goes down. And there are everyday spells that can easily kill you if you get them wrong, Apparition comes to mind. Yet it is possible that death in a magic accident is so common that it isn't even viewed as big tragedy. You'd have to be either extremely good (Dumbledore), or very careful (Moody, McGonagall), or both to survive to an old age.