Cat Fully Grown Age. Your cat will hit several different milestones that signify that shes becoming an adult cat, but theres no one magic age where a cat stops growing and maturing. While the average housecat is typically fully grown by the time it is 2 years old, giant breeds such as the maine coon might need up to 4 years to reach their final size.
At this age, the kitten becomes sexually mature and ready to start procreation. However, some of the larger breeds only stop growing between 4 and 5 years. The larger breeds are still growing for a year or two, but the smaller breeds have bloomed into glorious adult cats by now.