(1) The stages of life, from birth to death, may seem controlled by biology. (2) However, the way we think about life’s stages is shaped by society. (3) During the Middle Ages, for example, children dressed—and were expected to act—just like little adults. (4) Adolescence became a distinct stage of life only fairly recently, when a separate teenage subculture began to appear. (5) But in the Middle Ages, young people were “children” until about age 16. (6) Then they went to work, married, and had their own children. (7) Today, “young adulthood” has become a new stage of life, stretching from about age 20 to 30. (8) As life expectancy becomes longer and people spend years in active retirement, older adulthood has also become a distinct life stage. The sentence that expresses the main idea is: (Type the number of the sentence. Then click “GO.”)