Though the stereotype of feeding cats a saucer of milk may have influenced the way some humans feed the cats they encounter, adult cats actually cannot properly digest the lactose in dairy products such as milk and cream, making it a bad idea to let an adult cat drink milk. Not only will this lactose intolerance likely cause some discomfort for the cat, but it will also most likely result in diarrhea, vomiting or other symptoms of an upset stomach, which can be unpleasant to clean up. However, even though adult cats are lactose intolerant, this doesn't stop the furry felines from wanting to drink milk, so it is best for cat owners to make sure their pets don't have access to it.
Though it is not a good idea to give milk and other dairy products to adult cats, kittens are another story. As mammals, kittens are designed to consume their mother's milk. As cats reach adulthood, though, their dietary needs change, meaning that milk is no longer a suitable meal for a growing cat. However, some cats may retain the ability to digest lactose as they age, meaning these particular cats can handle the occasional dairy treat that other cats may not be able to handle.