Boxers are large, muscular, square-headed dogs who look imposing — that is, until you look into their eyes and see the mischief and joy of life reflected there. Because of their playful nature and boundless energy, they are sometimes called the “Peter Pan” of the dog breeds. 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 loves to play with them, but he’s also headstrong, especially if you try to use harsh training methods with him.