FIGURE 4 Distribution of the population of the United States by age, race, and Hispanic origin in 1990 [Bureau of the Census (1990); Census of Population; General Population Characteristics (1990); Census of Population CP-1-1;17-24].

to the size of the older population. The success of child survival programs and comparative failure of family planning programs in sub-Saharan Africa and South Asia have led to an unanticipated stage in the epidemiologic transition (49), the epidemic of communicable diseases of adolescents and young adults (i.e., STDs/AIDS).

Figure 4 shows the age distribution of the population of the United States in 1990 for blacks, Hispanics of any race, and non-Hispanic whites. The proportion of blacks and Hispanics from 5 to 24 years old substantially exceeds that of whites, so that the age pyramid for these U.S. minorities lies intermediate between that of the developing regions and that of industrialized countries.

Ages of Sexual Maturation and of Marriage

The average age of sexual maturation declined steadily during the 19th and 20th century in industrialized countries. In the Nordic countries, average age at menarche was about 16 years in the mid to late 19th century, falling steadily to 13 by the mid 20th century (50). Menarche generally occurs later in developing countries, but earlier in urban than in rural areas. This progressively earlier sexual maturation and the progressively delayed mean age of marriage greatly extends the duration of time that premarital intercourse can occur, making social, cultural, and religious proscriptions less effective than in the past.

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