When it comes to tooth replacement, cats will start to lose their teeth when they are four months old and will complete it after a period of time. When a cat's teeth change, the molars will be pushed out by new teeth, starting with the incisors and gradually replacing them. Kittens change teeth very quickly and will be replaced when they are five months old. If the owner observes carefully, he will find that cats in the teeth changing period will like to chew on toys.