At about 1 year old.

The exact age depends on the environment, nutritional status, breed and individual genetics. Plus, keep in mind that all living creatures keep growing to some extent, all our lives. Among mammals, our hair keeps growing; and cat's claws never stop growing. New synapses and brain connections are made all the time, too, in all but the most elderly mammals.

But otherwise, you can consider moggie a cat and no longer a kitten, from about 1 year old. If you're worried that your cat will never stop growing, the biggest and fattest cats on record have been a bit over 40 lbs in weight. The average size cat is about 10 lbs in weight.

Cats can start breeding, though, from about six months old, long before they stop growing themselves.