How old is "old"?  The answer has changed over the years. Two hundred years ago,you were old at 35.At the beginning of the 20 th century,the average life span was 45. In 1950,70-year-olds were really old.Today,a healthy 70-year-old is still thought young. So,how old is old? The answer is one you've heard many times,from all kinds of people."You are as old(or young)as you feel."Your age simply tells you how many years you have lived.Your body tells you how well you've lived. "Nobody grows old by living a number of years." wrote a writer."People grow old when they don't have their ideas." People shouldn't have the wrong ideas about _ Sometimes,older minds can be as bright as young minds.Alice Brophy once said,"It makes me unhappy when people say,'You look young for your age. ' What does that mean?You know you can die old at 30 and live young at 80. " The passage is mainly about   _   .
the average life span aging the 20th century older people and young people