单选题：Forty May Be the New 30 As Scientists Redefine Age　　Is 40 re

　Forty May Be the New 30 As Scientists Redefine Age　　Is 40 really the new 30? In many ways people today act younger than their parents did at the same age.
　　Scientists have defined a new age concept and believe it could explain why　populations are aging, but at the same time seem to be getting younger.
　　Instead of measuring, aging by how long people have lived, the scientists have factored in how many more years people can probably still look forward to. 　　Using that measure, the average person can get younger in the sense that he or　she can have even more years to live as time goes on," said Warren Sanderson of the　University of New York in Stony Brook.
　　He and Sergei Scherbov of the Vienna Institute of Demography (人口统计学)at the　Austrian (奥地利的) Academy of Sciences, have used their method to estimate how the　proportion of elderly people in Germany, Japan and the United States will change in the　future.
　　The average German was 39.9 years old in 2000 and could plan to live for another　39.2 years, according to research reported in the journal Nature on Wednesday.　However, by 2050 the average German will be 51.9 years old and will be expected　to live another 37.1 years. So middle age in 2050 would occur at around 52 years instead of 40 years as in 2000.
　　"As people have more and more years to live they have to save more and plan more　and they effectively are behaving as if they were younger," said nderson.
Five years ago, the average American was 35.3 years old and could plan for 43.5 more years of life. By 2050, the researchers estimate it will increase to 41.7 years and 45.8 future years.
　　"A lot o; our skills, our education, our savings and the way we deal with our health care depend a great deal on how many years we have to live," said Sanderson.
　　This dimension of how many years people have to live has been completely ignored in the discussion of aging so far. People 40 years of age today seem to be as young as A．their parents were at the same ag
B．their parents were at the age of 30.
C．their children will be at the same ag
D．their children will be at the age of 30.