Cats are considered adults typically by a year of age, and they are growing so fast that usually by 8-10 months theyre full-sized, sometimes they fill out a little bit more later. Certainly by the age of 2, they are pretty much full adults. However, once cats are fully developed physically and behaviorally, from about 3 years of age, for many years their outward appearance changes very little. Despite the youthful outward appearance, inside the cat will be getting older. It is much easier to understand how to manage cats’ lives, health, and behavior if we can associate their age with an equivalent of our own human years. The American Animal Hospital Association (AAHA) has designated four age-related life stages for cats: kitten (birth up to 1 year), young adult (1 year through 6 years), mature adult (7 to 10 years), and senior (aged over 10 years). The fifth, end-of-life stage can occur at any age.