How long can human beings live? Most scientists who study old age think that the human body is( 1)to live no longer than 120 years. (2),110 years is probably the longest that anyone could hope to live—if he or she is extremely healthy and lucky. Some scientists even say we can live as long as 130 years! Yet, our cells simply cannot reproduce( 3).They wear out, and as a result, we get old and (4)die. Even though we can't live forever, we are living a( 5)life than ever before. In 1900, the average American lifespan(寿命) was only 47 years, but today it is 75 years! When does old age begin then? Sixty-five may be out of date as the( 6) line between middle age and old age. After all, many older people don't begin to experience physical and mental decline( 7) they are after the age of 75. People are living longer because more people( 8)childhood. Before modern medicine changed the laws of nature, many children died( 9)common childhood disease. Now the chances of dying (10 )are much lower and the chances of living long are much higher due to better diets and health care. On the whole, our population is getting older. The (11)in our population will have lasting effects on our social development and our way of life. Some people fear such changes will be for the worse,(12)some see chances, not disasters. Many men and women in their "golden years" are healthy, still active, and young in(13 )though not at a young age. As the society grows old, we need the contribution of our older citizens. (14)long lives ahead of them,they need to( 15) active and dedicated.