A lot of current research into human longevity shows a relationship between being thin and living a long life. Most cats fed junk food (dry and canned commercial diets) are overfed and obese. They are sick and die young. Theophanes raises a good point--cats should be on a raw diet. They should also be kept lean, and have plenty of exercise.
Keep them indoors. An outside cat averages a 2 year lifespan... wildlife, cars, access to diseases from other cats... it's not a good thing. Other than that feed them a raw meat diet if you can. Kibble might be convenient but it's not natural or whole and the processing and chemicals added to it can cause a lot of problems... diabetes, UTI's, IBS, you name it. Other than that helpful hints include a large clean environment, another kitty friend (if they'll accept it) and toys to stimulate their minds (it helps prevent senility.) That all being said not all cats are going to last a long time. A lot of this depends on their genes. Some breeds, on average, just live a lot longer than others and the run of the mill mutt cat can be a real toss-up. You might have great luck, or terrible luck, or somewhere in between!
Yes, I agree cats can live a long and healthy life on a commercial diet. My friends' cat lived to 23 eating regular cat chow, although she did develop diabetes later in life. That cat lived indoors/outdoors for many years, and managed to avoid predators. Vet visits are important if a cat is ever injured or sick. I have two cats 14 and 15 years old that are thriving (on a commercial diet of wet and dry cat food), but recently lost two to dogs who are allowed to run loose in the neighborhood.
I moved from Pennsylvania to Oklahoma a few months ago. I've been planning on bringing my cat out here. A short time ago, my mom told me that it might not be a good idea to bring him out here because he is...