the stew does not I repeat does not keep them living longer it makes the weakened ones strong again which results in them being able to survive old age. It is all in the way the game is I keep them checked for easy tasks too like farming.... I noticed the ones dying were weakened beyond repair, when i found that special stew I tried it and it prolonged their early demise {for me anyhow}. and yes a villager can die from a weakened condition if they are not put on an easy task or allowed to recouperate from their injuries. It was just an observation I had made about the stew if I am wrong then I am or it is just sheer luck I do not know.