Measuring Human Aging
Despite their different backgrounds, researchers who study aging are challenged by the problem of how to measure it. While geological deep time is measured in large-scale epochs and eras, biological aging is calculated in maturational stages within specific life spans. Life spans represent longevity limits that are rarely achieved. The scientific community has set the human life span at 120 years. Life expectancy is the statistical figure based on the average person's length of life. In developed countries, medical advances and improved diet have allowed people to live longer and in greater numbers. Gerontologist Bernice Neugarten has divided the aging population itself into young-old and old-old categories to indicate this development. As the age curve lengthens, however, so do the possible number of diseases and incapacities suffered in later life. At the same time, in developing countries, poverty and the consequences of global inequality continue to undermine healthy populational aging.
Social aging is often measured in terms of ages or stages of life. For example, many African societies use complex and ritualized age-grade systems to identify the passages of life. Medieval European scholars mapped out seven ages of life according to a planetary model, beginning at birth with the moon and ending in old age with Saturn. Shakespeare's character Jaques, in As You Like It, articulated a memorable version of this model, making each age into a theatrical role: the infant, schoolboy, lover, soldier, justice, middle age, and old age—the "last scene of all . . . second childishness and mere oblivion, sans teeth, sans eyes, sans taste, sans everything." (act 2, scene 7) Ages-of-life models became superseded in the nineteenth and twentieth centuries by the sciences of aging, particularly developmental psychology, geriatrics, and gerontology. These sciences increasingly associated aging with the second half of life, or later life, which follows early-life processes of maturation and socialization. In psychology, G. Stanley Hall pioneered aging studies of both early and later life with his two influential books, Adolescence (1904), and Senescence (1922). Erik H. Erikson, a more contemporary developmental psychologist, theorized eight stages of development across the life cycle. He marked each stage by psychosocial modes of growth, crises, and resolutions centered around identity. For instance, in young adulthood (stage 6), the antithesis between generativity and self-absorption creates care. In old age (stage 8), the antithesis between integrity and despair creates wisdom (Erikson 1982). However, the work of Erikson and others in developmental psychology has been criticized by cross-cultural and feminist psychologists for its individualistic, ethnocentric, and masculinist models of the life cycle.
Geriatrics and gerontology emerged as fields of study in the early twentieth century by borrowing the expertise generated in psychology, biology, and medicine. Geriatrics and gerontology introduced two lasting contributions to the measurement of aging. First, aging and old age have their own physical, emotional, and psychological dynamics distinct from other stages of life. Second, aging and old age are best understood if disease pathologies and normal senile conditions are separated. Early clinicians such as Jean-Martin Charcot, Ignatz L. Nascher, Elie Metchnikoff, and Edmund V. Cowdry attributed the problems of aging to specific degenerative processes in the cells, tissues, and organs of the body. Gerontology grew apart from geriatrics in the later twentieth century to include sociological, demographic, and policy studies. Gerontologists also attacked traditionally ageist notions of decline with new, positive measurements of creativity, wisdom, and the benefits of aging. Gerontological research on positive measurements of successful aging has continued with criticism of the negation of aging stemming from modern culture's adulation of youthfulness.