You are absolutely right that he will most likely kill her right now. You have a couple of options. If you want these to be a pair, you need to wait at least 3 years before you introduce them into the same cage. It’s very dangerous for a young hen to be paired with an older male. He will be ready to mate long before she is. She might be sexually mature and even capable of laying eggs by a year old, but a responsible breeder would never allow this. In the wild they are over 3 before they pair up and breed. In captivity, we have to make sure birds are ready for breeding before we pair them up. Sadly many novice breeders will pair birds too early. Much like a human, they are sexually active long before they are physically or emotionally ready for eggs and chicks. Laying eggs takes a physical toll on a hen. It require a lot of calcium and protein to form each egg, and then there is the physical strain of the egg moving through her reproductive system and finally her effort to pass the egg. A young hen is at a high risk of becoming egg bound, which is most often fatal with a young bird. She doesn’t have the physical build yet to undergo that strain and effort, or the means to replenish her calcium and protein. While she appears fully grown, she is still going through some physical growth and development. And emotionally, she is hardly ready to take care of eggs and chicks. A male under 3 is eager to mate, but that’s about it. He doesn’t want to settle down to care for eggs or to have his mate do this. He is likely to break and/or eat the eggs in order to make her mate with him again. If she tries to protect or stay on the eggs, he may become aggressive towards her and injure or kill her. There can of course be exceptions, but for the best results, healthiest chicks and in the best interest of the health of the parents, it’s best for both birds to be a minimum of 3 before you even pair them in the same cage. If this is longer than you planned to wait, then try finding a female about the same age as the male, and then you only have to wait a year before you pair them up.
Thank you for asking Lafeber,