Villagers can become weakened and die for a variety of reasons, including starvation, old age, extended illness, animal attacks, and on-the-job injuries. Early in the game, you really need to make sure that your villagers are working on the basics: foraging to provide an immediate source of food, researching new ways to get food, and healing villagers that become sick. If you grow your tribe too quickly, your villagers may not have enough food available and starve to death.
You can sometimes help weakened villagers by having them work on less strenuous duties. Farming and medicine seem to be less hazardous occupations than building and research. Alas, for the elderly, even light duty may not prolong their life by much. Researching higher levels of medical technology does extend their lives significantly.
There are some helpful tips in the game's tutorial, which was offered when you first started Village Sim. You may also be interested in checking out the
Village Sim Guide section of the forums.
