So a question about rats' death

This could be stupid - can rats die in their sleep of old age without having been sick or weak-looking for any period of time? It doesn't bother me that my rats will die. Well, it does, but I can accept it. The thing that kills me is when they're all sick and weak-looking before they die. It's agonizing to me. I went through it with one of my girls last month and it's hard even imagining going through it again with 5 more rats - especially since my favorite is getting up there in age. I'd feel so much better if they simply died peacefully in their sleep when the time came but I don't know how common that is.