While an in-depth understanding of internal medicine serves as a foundation, proper care of older adults should be complemented by insight into the multidimensional effects of aging on disease manifestations, consequences, and response to treatment. In younger adults, individual diseases tend to have a more distinct pathophysiology with well-defined risk factors; the same diseases in older persons may have a less distinct pathophysiology and are often the result of failed homeostatic mechanisms. Causes and clinical manifestations are less specific and can vary widely between individuals. Therefore, the care of older patients demands an understanding of the effects of aging on human physiology and a broader perspective that incorporates geriatric syndromes, disability, social contexts, and goals of care. For example, care planning for the older patient should account for the substantial portion of the wide variability in life expectancy across individuals of the same age that can be predicted by simple and inexpensive measures such as walking speed. Estimation of the expected remaining years of life can guide recommendations about appropriate preventive and other long-term interventions and can shape discussions about treatment alternatives.
(See also Chap. 93e) Population aging emerged as a worldwide phenomenon for the first time in history within the past century. Since aging influences many facets of life, governments and societies—as well as families and communities—now face new social and economic challenges that affect health care. Fig. 11-1 highlights recent and predicted changes in U.S. population structure. The overall number of children has remained relatively stable, but explosive growth has occurred among older populations. The percentage of growth is particularly dramatic among the oldest of the old. For example, the number of persons aged 80–89 years more than tripled between 1960 and 2010 and will increase over tenfold between 1960 and 2050. Women already outlive men by many years, and the sex discrepancy in longevity is projected to increase further in the future.
Population aging occurs at different rates in varying geographic regions of the world. Over the past century, Europe, Australia, and North America have had the populations with the greatest proportions of older persons, but the populations of Asia and South America are aging rapidly, and the population structure on these continents will resemble that of “older” countries by around 2050 (Fig. 11-2). Among older persons, the oldest old (those >80 years of age) are the fastest-growing segment of the population (Fig. 11-3), and the pace of population aging is projected to accelerate in most countries over the next 50 years. There is no evidence that the rate of population aging is decreasing.