Developmental Aging and Cognitive Processes Across the Lifespan

Introduction to Aging and Senescence

Aging is defined by the Merck Manual of Geriatrics as "a process of gradual and spontaneous change resulting in maturation through childhood, puberty, and young adulthood and then decline through middle and late age." Aging is a subject that affects the individual both in thought and in fact, with many mixed feelings and emotions. There are positive aspects to the process of aging, such as the gaining of wisdom and experience, but there are also effects such as gray hair, baldness, and memory loss that are negative in nature and not desired by the human being.

Senescence is defined by the Merck Manual of Geriatrics as "the process by which the capacity for cell division, growth, and function is lost over time, ultimately leading to an incompatibility with life — i.e., the process of senescence terminates in death." Merck states that the differentiation between what is termed "normal aging" and "successful aging" is useful in defining certain aspects of the aging process.

Normal aging refers to the common factors, diseases, and impairments characterizing the elderly population. It is important to note that aging individuals age at different rates, acquire different diseases and impairments, and some remain so healthy that when they do die, they can be said to have died of nothing other than simply "old age." Successful or healthy aging references "a process by which deleterious effects are minimized, preserving function until senescence makes continued life impossible" (Merck Manual of Geriatrics, 2005). Successful aging, then, is aging that is not accompanied by debilitating disease and disability. Although the percentage and proportion of the elderly have increased in the United States, the percentage of elderly who reside in nursing home facilities has decreased by 5.2%.

The purpose of this study is to research aging from the aspect of cognitive development, which includes all aspects of adult development encompassing theory and research. This work takes a cognitive cohort perspective inclusive of case studies from both male and female perspectives. Phases of development include mental, physical, and emotional changes experienced by the aging individual. The three ranges covered in respect to developmental cohorts are the Middle Age range of 40–60 years of age, the Older Mature phase of 60–80 years of age, and the Elder range of ages 80 and beyond.

Aging and senescence both witness a decline in physiological functions; however, normal decline is not thought of as anything other than a "normal decline." Cognitive decline is said to be experienced on a universal basis with advanced age and is within the scope of normal aging, but cognitive decline is stated to be "consistent with dementia" even though it is common in the later part of an individual's life. Alzheimer's disease is a "pathological process distinct from normal aging, a conclusion that is supported through analysis of brain tissue at autopsy" (Merck Manual of Geriatrics, 2005).

The life span refers to the average number of years that an individual is expected to live under normal conditions. The maximum life span for women is stated to be approximately 125 years (Merck Manual of Geriatrics, 2005), and somewhat shorter for men. The factors that affect longevity are primarily hereditary aspects that determine the likelihood of an individual contracting disease or inheriting other medical conditions.

Biological Foundations of Aging

Unless they become cancerous, cells eventually lose the ability to divide, which limits the replicative capacity of cells. Cells that have divided so many times that they are rendered unable to divide again will enlarge and exist for a time before dying. The reason cells become unable to divide is understood through the DNA process. DNA contains telomeres at the ends of chromosomes, which serve as a sort of handle for the movement of chromosomes during the telophase of meiosis. These telomeres are shortened each time a cell divides, and when they become too short, the cells lose the ability to divide. When cells are transformed into cancerous cells, the enzyme telomerase lengthens the telomeres, so the telomeres of transformed cells no longer shorten at cell division, making the cells effectively immortal (Merck Manual of Geriatrics).

There may be other mechanisms involved in senescence beyond telomere shortening, such as messenger RNA transfers from senescent cells into younger cells that stop division in the younger cells. Necrosis or apoptosis may also occur from cell death, generally brought about by chemical or physical insult that "overwhelm normal cellular processes and make the cells become nonviable" (Merck Manual of Geriatrics, 2005).

Necrosis is defined as a "purely entropic phenomenon due to loss of the cell's ability to transform external energy" (Merck Manual of Geriatrics, 2005). Apoptosis is defined as a "highly regulated and orderly process by which a cell effectively commits suicide" (Merck Manual of Geriatrics, 2005). Apoptosis has been linked to several age-related diseases, such as Alzheimer's disease.

Aging has many different variables and occurs at rates that differ among individuals and among species. Senescence is viewed by gerontologists as a "collection of degenerative entropic processes related only by the fact that they occur over a period of time." Several theoretical models have been proposed to explain the mechanisms of senescence:

The Loose Cannon Theory proposes that an entropy-producing agent — a free radical or glucose — disrupts cellular macromolecular constituents over time. The Rate of Living Theory proposes that smaller animals have higher metabolic rates and therefore tend to die at a younger age than larger mammals. The Weak Link Theory proposes that a specific physiologic system — such as the neuroendocrine or immunological system — is more vulnerable during senescence, and upon failure of this system, the deterioration of the entire body accelerates.

The Error Catastrophe Theory proposes that DNA or RNA errors of transcription lead to genetic-based errors that promote senescence; data suggest that older organisms have accumulated altered proteins reflecting such genetic changes. The Master Clock Theory proposes that aging is under direct genetic control. This theory is considered the oldest of all aging theories and is no longer regarded as viable. In fact, whatever actually controls the process of aging remains unknown.

Progeroid syndromes are rare diseases that exhibit several features similar to those normally observed in the elderly population, including baldness, osteoporosis, and dry and wrinkled skin. Several specific syndromes are notable:

Werner's Syndrome produces sclerodermal skin changes and baldness, as well as premature cataracts, muscular atrophy, glucose intolerance, a high incidence of cancer, and early death attributable to arteriosclerosis.

Wiedemann-Rautenstrauch Syndrome produces premature scleroderma, baldness, and other senile pathologies in children. Its genetic basis is yet undetermined.

Hutchinson-Gilford Syndrome similarly produces premature scleroderma, baldness, and other senile pathologies in children, also with an undetermined genetic basis.

Down Syndrome is more common than the progeroid syndromes. It produces pathologies typical of senescence, including glucose intolerance, vascular disease, a high incidence of hair loss, degenerative bone disease, and premature death. The central nervous system is impaired, generally producing intellectual disability while also accelerating the accumulation of neuritic plaques and neurofibrillary tangles characteristic of Alzheimer's disease. Chromosome 21 mutations have been implicated in Alzheimer's disease, but the specific gene related to Down syndrome is yet undetermined.

Psychological and Developmental Theories

The study of developmental psychology focuses on the changes of individuals over time and the processes that bring about those changes. Two main processes cause individuals to change during their lifetimes: (1) maturation and (2) learning. Maturation is defined as the developmental changes that occur as a result of the aging process; this information is encoded in the genes of the individual. Learning is a relatively permanent behavioral change that results from practice or experience.

Cohorts are groups of people who share similar life experiences. For example, age differences among groups of people of different ages could be the result of cohort differences, which arise from cultural and historical factors encountered during development and growth.

The body reaches its peak efficiency at approximately age 30 and then begins its decline. Using age 20 as reflective of 100% performance (Graham, 2005), the following changes are observed:

The last areas of the brain to complete development cause cognitive development in humans to lag behind physical development. The desire to understand the world around us drives cognitive development. Jean Piaget, a Swiss theorist, attained his PhD at the age of 21 and worked at Alfred Binet's school laboratory, where the first testing of intellectual abilities was developed. Piaget was intrigued by how children answered questions incorrectly, and through studies of his own children he expanded his research to larger groups of children, making a name for himself in the United States by the 1960s, though he had been well known throughout Europe since the 1930s. Piaget proposed that people create mental models — or schemas — about how the world works.

Piaget also postulated two general processes governing schema change:

According to Piaget's theories, there are four stages of human cognitive development. He suggested that the stages occur in an "invariant developmental sequence" and that the exact ages at which children develop and move to the next stage differ from one child to another:

The theory of personality proposed by Abraham Maslow has influenced several fields. Maslow was a humanistic psychologist who held that humans are not simply pulled and pushed by mechanical forces, and the humanist perspective focuses upon human potentials. Maslow set out the hierarchic theory of needs, which are instinctual and based on five levels of basic needs:

Gardner's theory of multiple intelligences identifies the following forms of intelligence: