Complex sentence: People congregated to the cities and created shaky yet stable societies, but every person born after that day is no longer capable of bearing children, and the world now knows no human being younger than 17 years old.
Split into simpler sentences: People congregated to the cities and created shaky yet stable societies. Every person born after that day is no longer capable of bearing children, and the world now knows no human being younger than 17 years old.