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  Chapter One:

  Size and Growth Rate
  Life Expectancy
  Age Structure
  Population Drift
  Urban, Rural, Suburban
  Foreign Born
  Large Cities


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Age Structure

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The proportion of children and adolescents in the population declined, while the proportion of older people increased throughout the century.
These two phenomena follow mechanically from the falling birth rate and rising average length of life. As the birth rate falls, the ratio of children to adults necessarily diminishes and the average age of the population rises. As people live longer on average, the proportion of the population at older ages necessarily becomes larger. 

Because the decline in the birth rate was almost continuous (with the exception of the baby boom) and the lengthening of lifetimes fully continuous, the proportion of children and adolescents in the population decreased steadily from 44 percent in 1900 to 29 percent in 1998. If the birth rate declines further or remains stable and average lifetimes continue to lengthen, the youthful component of the population will continue to decrease. The Census Bureau’s middle series projection indicates that children and adolescents will constitute barely a fifth of the population by 2020. 

These changes at both ends of the age spectrum did not have much impact on the relative size of the intermediate group between the ages of twenty and fifty-nine. This segment represented roughly 50 percent of the population throughout the twentieth century, and this is not expected to change much in the twenty-first. That percentage is important because it represents a ratio of 1:1 between people of working age, the great majority of whom are economically active, and their individual or collective dependents.

Chapter 1 chart 3

Source Notes
Source Abbreviations

HS series A 119–134. See also CB, Population Estimates Program, Population Division, “Resident Population Estimates of the United States by Age and Sex” at (accessed August 20, 2000).


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