According to Web MD, the average life expectancy for house cats is about 15 years. However, some cats can live to be over 20 years old. A given cat's longevity is influenced by genetics, diet, environment and luck.
Many of the same factors that affect humans' health also play a role for cats. Web MD points out that cats that maintain a healthy weight are likely to live longer, as are those that are more active. While cats rather naturally adopt sedentary lifestyles, those who get regular exercise are better able to stave off disease and old age.
The ASPCA lists several common foods that are bad for cats and can make them sick. Among these are onions, garlic, raisins, coffee and chocolate. Additionally, cats are very sensitive to salt, so salty foods should be limited.