Doctors are of the opinion that most people cannot live beyond 100 years, but a growing number of scientists believe that the aging (衰老) process can be controlled. There are more than 12,000 Americans over 100 years old who receive Social Security (社会保险) benefits, and their numbers are increasing each year. Dr. James Langley of Chicago claims that, theoretically and under ideal conditions, animals, including man, can live six times longer than their normal period of growth. A person's period of growth lasts approximately twenty-five years. If Dr. Langley's theory is accurate, future generations can expect a life span(寿命) of 150 years. Which of the following statements best expresses the main idea of the passage?