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.
[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.