Everybody knows that old people are more likely to die than young people. We know it from experience. We also can find the facts and figures in life insurance tables. For example, in an advanced country with a well-fed population only about one in every 1. 000 children dies during his or her 12th year. But one in every 20 men aged 70 will die before his 71st birthday.

The risk we run of dying at any particular age is called the force of mortalityfor that age. The force of mortality has been carefully worked out for all ages of people-this is what life insurancerates are based on. Naturally, it grows steadily greater with age. If we kept all through life the same force of mortality  that we had at 12-never becoming any more likely to die than we were then-we could all hope to live several hundred years, unless we were very unlucky or careless. The fact that people do not live several hundred years (and only rarely reach 100) is due to a process called aging.

We can recognize by the gray hair, weakened muscles, wrinkled skin, loss of hearing, and other signs that it produces. It also has a more important effect on us. It reduces our power of staying well and of getting better if we fall ill. A common cold may not be serious in a young person, but its complications may lead to death in an old person.

This loss of the power to stay healthy (and the increase in the likelihood of dying) happens at about the same rate in everybody. As we age, we tend to be like an old automobile-more things go wrong with us. But old cars and radios do not repair themselves. When we are young, our bodies do. What seems to decline with age is the power of self-repair. There comes a time beyond which it is very difficult to stay alive at all. The least thing may be enough to finish us. This is the end of our life span.