Cats typically are full grown by their first birthday. Their bones stop growing by eight months, but sexual maturation takes another four months. During the second year, a cat's frame fills out, but his bone structure is set.
Growth speed is slower in some breeds. Larger breeds like Savannah and Maine Coon, for example, take three to five years to reach their full growth and often weigh 30 pounds or more. Just like humans, cat genetics play a big part in the size of a cat. A cat may stop growing or grow more slowly if their nutritional requirements are lacking.