Consideration of the teenage years as a separate stage of life did not occur until many decades after childhood became recognized and treated as a distinct period of human development. As the demand for technically and professionally trained individuals grew and children began remaining in school for many more years than formerly, a kind of subculture began to form. Individuals in these groups, who were between 12 and 20 years of age, had their own interests, attitudes, values, and behaviors, which often led to consternation on the part of the dominant adult community and the characterization of adolescence as a period of "storm and stress!" The publication in 1904 of G. Stanley Hall's two-volume treatise on Adolescence is generally viewed as the beginning of the field of adolescent psychology. Hall was also a pioneer in another developmental field, that of gerontology—the study of all aspects of aging and later life. Hall's Senescence: The Last Half of Life was the first published book on this topic in the United States and one of the first in the world.
Life expectancy—the number of years that a group of people born in a given year can be expected to live—and the median age of the population have increased markedly during this century. These increases, in addition to declines in infant mortality, have been associated with medical advances in the treatment of infectious disease and in improvements in sanitation, nutrition, and overall health care. One consequence of the "greying of America," reflected in the greater number of older adults, has been increased political and economic power of this segment of the population.
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