We went with the whole "you won't die until you're very old" thing too, but both DD1 and 2 heard things on the news, had a family friend who died and so we added that sometimes very sick people die too. DH and I are not at all religious, but have chosen to send the girls to a private Christian school. As a result, we also tell the girls that when you die you go to heaven, along with family dogs, cats etc. It ties in with the education and belief system we have chosen to support, but most of all it seems to give them enormous comfort that anyone who dies won't be by themselves ITMS.