While I haven't figured out how to make an elderly villager live longer, I have figured out a use for the females. Since the elderly villagers walk slower than the others, they tend to take longer doing most tasks. So if you are in need of babies, use the Elderly women. A nursing mother will do nothing else anyway, so you might as well give the slower villagers the task of motherhood. That saves your younger, faster females for farming, building, etc.
Added by: SkyeLark Aug 24th 2009, ID#14262