The usual story is that people lose muscle as they age due to disuse. But more and more research I come across shows that nutrition may be even more important than exercise when it comes to maintaining muscle mass. Here's a sampling:
Alkaline diets (high in fruits and vegetables) favor maintenance of lean mass
So if you have all your nutritional ducks in a row, how much exercise and resistance training is needed to stave off muscle loss during aging? I don't know the answer, but I think it's an interesting question - one that future research will hopefully be directed towards.