Living indoors is only one of the factors that affects longevity. Other factors that affect longevity include genetics, vaccines and medications, what your cat eats, and what chemicals you use in the household. If everything is ideal it would be realistic to hope your cat will hit 20 years old. However, many of our cats are bred from cats that have had 1 million things going on in the last several generations and have not lived anywhere near this long. That is one factor. If your cat is all indoors and yet you go for annual vaccinations year after year for your cat, then making it to 20 years old would be a surprise. If your cat eats meat his or her entire life and does not eat dry food based on corn, byproducts and other inferior quality ingredients, then chances are good your cat will approach 20. If you are a smoker, if you use air fresheners and a lot of cleaning products then you may be reducing your cats life expectancy. I would love to see all of our cats live to 20+ years old!