So what I am guessing from the comments on the other thread is that the game decides when they are going to die, and you can only influence it by 1-3 years if you are giving them healthy products from the store and encouraging healthy behaviors.

I might also mention that now that they can afford it, I no longer buy anything but organic foods. That doesn't seem to have had any effect on life expectancy either, but they don't seem to get serious infections anymore. The most we seem to get are rashes, hiccups, and stomach aches.