A、4%. B、10%. C、14%. D、25%. B细节题。短文开头提到，如今每100个美国人中就有10个人超过65岁，由此可知现在美国大约有10％的人口超过65岁，故答案为B)。

America is growing older. Fifty years ago, only 4 out of every 100 people in the United States were 65 or older.[19]Today, 10 out of every 100 American’s are over 65. The aging of the population will affect American society in many ways — education, medicine and business.
    Quietly, the greying of America has made us a very different society —one in which people have a quite different idea of what kind of behaviour is suitable at various ages.[20]A person’s age no longer tells you anything about his/her social position, marriage or health. There’s no longer a particular year in which one goes to school or goes to work or gets married or starts a family. The social clock that kept us on time and told us when to go to school, get a job, or stop working isn’t as strong as it used to be. It doesn’t surprise us to hear of a 29-year-old university president or a 35-year-old grandmother, or a 70-year-old man who has become a father for the first time.
    Public ideas are changing. Many people say, "I’m much younger than my mother or my father was at my age." No one says "Act your age" anymore. We’ve stopped looking with surprise at older people who act in youthful ways.
19. What is the percentage of people above 65 in America today?
20. What effect does the greying of America have on its society according to the passage?
21. What does the passage imply?