Doctors are of the opinion that most people cannot live beyond 100 years, but a growing number of scientists believe that the aging process can be controlled. There are more than 12,000 Americans over 100 years old who receive Social Security benefits, and their numbers are increasing each year. Dr. James Langley of Chicago says 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 nearly twenty-five years. If Dr. Langley's theory is accurate, future generations can expect a life span of 150 years. The main idea of this passage deals with .