Boxers originated in Germany and were brought to the U.S. after World War I. Boxers are large, muscular, square-headed dogs. They are playful and energetic. Boxers aren't considered fully mature until they are three years old, meaning they have one of the longest puppyhoods in the world of dogs. The typical Boxer is intelligent, alert, and fearless, yet friendly. He's loyal to his family and friendly with children. He's also headstrong, especially if you try to use harsh training methods with him.