Early twenties family formation bring the norm was more of a postwar thing. The guys that came back from the war really did have to grow up fast (seeing your best friends getting blown up at 18 will do that), and they essentially had zero desire to have racous twenties filled with dating around and traveling and soul searching. They'd had enough chaos already, and were all extremely eager to settle down into a peaceful family life immediately upon their return home. The age of family formation has slowly crept back upward since then, and historically, in normal peaceful times it's usually been late twenties.