The distinction between childhood and adulthood varies considerably among cultural and social groups and across historical periods. Aging is not only a biological process; it is also a social process. The personal and social significance of the passage of years is shaped by the cultural age system. All societies divide the life span into recognized stages. These life stages or periods are marked by certain physical, psychological, and/or social milestones. Privileges, obligations, rights, and roles are assigned according to culturally shared definitions of periods of life (Fry 1980; Hagestad and Neugarten 1990). In Western industrialized societies, the life stages are commonly identified as: prenatal stage (from conception until birth); infancy (from birth to the end of the second year of life); early childhood (ages three to six years); middle childhood (six years until puberty); adolescence (start of puberty to adulthood); young adulthood (ages twenty to forty); middle adulthood (ages forty to sixty-five); and later adulthood or old age (sixty-five and older).
These socially constructed life stages are not fixed; rather, they have expanded and contracted in length and new ones have emerged in response to broader social changes. For example, Jeffrey Jensen Arnett (2000) proposes emerging adulthood as a new conception of development for the period from the late teenage years through the twenties (with a focus on the ages of eighteen to twenty-five) in industrialized societies that allow young people a prolonged period of independent role exploration (Arnett 2000). In doing so, he draws parallels between his conception of emerging adulthood to Erik Erikson's concept of prolonged adolescence in industrialized society in which "young adults through role experimentation may find a niche in some section of his society" (Erikson 1968, p. 156).
Arnett (2000) argues that emerging adulthood is distinct demographically. It is the "only period of life in which nothing is normative demographically" (p. 471). Almost all of U.S. adolescents from twelve to seventeen years of age live at home with one or more parents, are enrolled in school, and are unmarried and childless. In contrast, emerging adults' lives are characterized by diversity. About one-third of young persons in the United States go off to college after high school, another 40 percent move out of their parental home for independent living and work, and about 10 percent of men and 30 percent of women remain at home until marriage. About two-thirds of emerging adults experience a period of cohabitation with an intimate partner (Michael et al. 1995). These emerging adults often change residences, including temporarily moving back into their parents' home. It is estimated that about 40 percent of recent cohorts of young adults have returned to their parent's home after moving away (Goldscheider and Goldscheider 1994). Arnett notes that it is with the transition to young adulthood, as more stable choices in love and work are made, that the diversity narrows. As further evidence of emerging adulthood as a distinct life stage, Arnett (2000) cites a survey in which the majority of people in the United States in their late teens and early twenties indicated "somewhat yes and somewhat no" versus an absolute "yes" or "no" to whether they felt they had reached adulthood.
Changes are occurring not only in the social construction of entry to adulthood but also in the social conception of the late stage of adulthood, or old age. The definition of old age as beginning at age sixty-five is a relatively recent phenomenon. It reflects primarily the decision of European countries and the United States, in their creation of their old-age social insurance programs (i.e., national retirement or pension systems) during the first half of the twentieth century, to establish this chronological age as determining eligibility. More recently, the growing numbers of older adults, especially those age eighty and older, has resulted in the redefining of later adulthood into the two distinct life stages or age groups: the younger-old (ages sixty-five to seventy-five) and the older-old or oldest-old (older than seventy-five). Indeed, in many countries, the oldest-old are now the fastest growing portion of the total population. Persons aged eighty and older represented 17 percent of the world's elderly in 2000: 23 percent in developed countries and 13 percent in developing countries (U.S. Census Bureau 2000b). Stressing the relative newness of the oldest-old phenomenon, Richard Suzman and Matilda White Riley (1985, p. 177) emphasized that "less is known about it than any other age group" and that there "is little in historical experience that can help in interpreting it."
Approximately a decade later, similar claims were asserted about middle age. Orville G. Brim, Jr. (1992, p. 171) referred to the middle years as the "last uncharted territory in human development." Concern about the status and welfare of children and the elderly contributed to the scientific study of the biological, psychological, and social development of these two vulnerable populations. This concern led to the enactment of federal and state statutes to protect children and elders. Many researchers' lack of interest in the midlife reflected the predominant view that personality is stable during adulthood. Moreover, from a public policy perspective, adults were not viewed as a vulnerable population requiring protection of "their best interests" by the state.
Bernice L. Neugarten and Nancy Datan (1974) noted that researchers and clinicians constructed dissimilar images of the understudied middle years of adulthood. Whereas researchers characterized young adulthood as a period of major transitions, middle adulthood was often viewed as a plateau with little of significance occurring until old age. In contrast, clinicians often portrayed middle age as a time of crisis. The aging of the baby boomer generation and the sheer number of this cohort entering midlife have had a profound impact on the current interest in this life stage. In 2000 more than 80 million members of the baby boomer generation were between the ages of thirty-five and fifty-four (U.S. Census Bureau 2000a). The interest in middle age spurred the MacArthur Foundation Research Network on Successful Midlife Development (MIDMAC); one of the most significant inter-disciplinary research endeavors devoted to the study of midlife. The studies emerging from the MIDMAC large representative survey of midlife in the United States are reshaping our understanding of these middle years. It was only in 2001 that the first Handbook of Midlife Development, one of the most significant contributions to the field, was published (Lachman 2001).
The period called middle age lacks well-defined boundaries. Michael P. Farrell and Stanley Rosenberg (1981, p. 16) note "like defining a period of history, no one quite agrees when middle age begins or ends." Margie E. Lachman and her colleagues (1994) found that the subjective boundaries of middle age vary positively with age. The older an individual is, the more likely she or he will be to report later entry and exit years as demarcating middle age. Although the ages of forty to sixty are typically considered to be middle-aged, for some persons middle age begins as young as thirty and for others middle age is not perceived as ending until age seventy-five. Middle-aged persons typically report feeling about ten years younger than their chronological age (Montepare and Lachman 1989). As life expectancy increases, the boundaries of middle age may also shift. A National Council on Aging (2000) survey revealed that one-third of Americans in their seventies perceive of themselves as middle-aged. Midlife or middle age does not exist as a concept in all cultures; there is also considerable cultural variation in the social construction of this life stage. Usha Menon (2001) illustrates this variation through a comparative analysis of the conception of middle age and the social roles associated with this stage of life in three societies—middle-class Japan, upper-caste Hindu in rural India, and middle-class Anglo-America.
Whereas childhood and adolescence are often marked by formal rites of passage, the transition from young adult to middle-aged adult is marked neither by special rites of passage nor by predictable chronological events. The transition from young adulthood to middle adulthood is often a gradual one. Social cues, especially changes in family and work domains, may be better indicators of developmental change than chronological age alone.
The midlife research of the past decade has dispelled many of the myths and negative stereo-types of middle age. For most middle-aged adults, their physical health is good, although concerns are expressed about being overweight and future declining health (American Board of Family Practice 1990). Only 7 percent of adults in their early forties, 16 percent of adults in their early fifties, and 30 percent of adults in their early sixties have a disabling health condition (Bumpass and Aquilino 1995). Although middle-aged adults often face a number of family and work stresses, for both men and women there is evidence of a decrease in negative emotions and an increase in positive mood in the middle adult years (Mroczek and Kolarz 1998).
Despite the pervasive and persistent societal view of menopause as a stressful life experience, research has consistently documented that most women pass through menopause with little difficulty. In a longitudinal study of the menopausal transition, Nancy E. Avis and Sonja M. McKinley (1991) found that more than two-thirds of women report relief or neutral feelings about the cessation of menses and that over a four-year period changes in attitudes toward menopause are in a positive or neutral direction. Rather than a crisis, the majority of women viewed their post-menopausal period as a new and fulfilling stage of life. The loss of fertility in menopause is sometimes experienced as a gain in freedom in sexual expression. It is important to stress that there is wide cultural variation in the menopausal experience. For example, Japanese women do not view menopause as a distinct event or a disease; rather it is seen as part of the general aging process. Thus, the physiological changes Japanese women identify as associated with menopause—stiff shoulders, dizziness, headaches, and dry mouth—are changes identified more broadly with growing older. In contrast to U.S. women, few Japanese women identify hot flashes or sweats as symptoms of menopause (Lock 1994).