The "graying of America" has occurred for at least two reasons.
First, there is the fact that people are living longer than ever before. This is due to advances in medical science as well as to reductions in risky behaviors like smoking and driving without seat belts. When coupled with the large number of people in the "baby boom" this means that there are more and more elderly Americans.
Second, there is the fact that Americans are having fewer children than they previously did. This is due in part to advances in contraception. But it is also due to changing societal expectations about the role of women and changing views as to what size of family is ideal.
Since the baby boom generation is growing older and living longer, and since younger people are having fewer children, the U.S. is "graying," an allusion to loss of hair color.