题目内容：
回答题：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 timeseem to be getting younger.
Instead of measuring aging by how long people have lived,the scientists have factored in how many more years peoplecan probably still look forward to.
"Using that measure, the average person can get youngerin the sense that he or she can have even more years to liveas time goes on," said Warren Sanderson of the University ofNew York in Stony Brook.
He and Sergei Scherbov of the Vienna lnstitute ofDemography （人口统计学） at the Austrian （奥地利的） Academy ofSciences, have used their method to estimate how theproportion of elderly people in Germany, Japan and the UnitedStates will change in the future.
The average German was 39.9 years old in 2000 and couldplan to live for another 39.2 years, according to researchreported in the journal Nature on Wednesday.
However, by 2050 the average German will be 51.9 yearsold and will be expected to live another 37.1 years. Somiddle age in 2050 would occur at around 52 years instead of40 years as in 2000.
"As people have more and more years to live they have tosave more and plan more and they effectively are behaving asif they were younger," said Sanderson.
Five years ago, the average American was 35.3 years oldand could plan for 43.5 more years of life. By 2050, theresearchers estimate it will increase to 41.7 years and 45.8future years.
"A lot of our ski/Is, our education, our savings and theway we dear with our health care depend a great deal on howmany years we have to live," said Sanderson.
This dimension of how many years people have to live hasbeen 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.
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