China’s large labor force has been central to its rise as an economic power, allowing companies to tap a seemingly endless pool of workers willing to move from their home towns, often live at the factory site and accept comparatively low wages. That era is ending. In a shift that is intensifying the economic competition between China and the United States, China’s working-age population has plateaued in size and will begin getting smaller sometime in the next five years, according to demographers and recently released census data.