Which stage of lifespan development is not universal and only exists in industrialized or postindustrial countries?

The correct answer is emerging adulthood. 

Emerging adulthood refers to a stage of lifespan development that occurs during ages 18-25 approximately, where young adults undergo a period of discovering their identity, transitioning from adolescence to adulthood and finding their place in the world. Emerging adulthood only exists in industrialized or postindustrial countries, where marriage and having children usually occurs around age 30, and young adults are provided with a longer period of independence during their 20s.