Although there are various calculating methods to determine a cat's age in human years, I think this is a good one. According to this your cat is 85 years old.
http://www.calculatorcat.com/cats/cat-years.phtml
Even for people it is hard to keep weight on as one ages....how many fat 85 y.o. do you see? Not many, is my observation, most are pretty thin. Muscles weight more than fat, and as body ages, muscle tone is lost. If your cat is healthy and does not have a severe deteriorating condition, e.g. failing kidneys=drinking lots of water, your cat could live into it's 20s. My neighbour's cat just died at age 22. Take him to the vet and have his health evaluated. Let us know how it goes. All the best!
http://www.calculatorcat.com/cats/cat-years.phtml