Cats can eat a lot of foods that humans can including fish. While it's not safe to give your cat lots of human foods without consulting your veterinarian, most veterinarians will agree that fish is a good option for cats at almost every stage of life.
Fish supplies cats with essential amino acids that they need to survive. Fish helps to build up their joints and ensure that your cat's brain is working properly.
Oily fish is especially good for your cat. Consider feeding them fish such as mackerel and tuna.