Perhaps two of the greatest human achievements to have occurred from prehistoric to more recent times, and, particularly, in the last two centuries, are the increase in the human population worldwide and the extension of human longevity. Thanks to improved living conditions stemming from advances in agricultural practices and advances in industry and technology, the human population today has reached an unprecedented size (1-3). Living conditions have greatly improved due to
1. public health reforms and improved personal hygiene,
2. advances in medical knowledge and practices,
3. vastly increased control over the environment, and
Never in human history have so many humans lived longer: In the United States, for example, the percentage of Americans aged 65 and older, relative to the entire U.S. population, rose from 4% in 1900 to 8% in 1950 and to 13% in 1990, and these increases are even greater in the population statistics of other countries (Chapter 2). Indeed, because of this longevity revolution that owes much to improvements in technology and extraordinary advances in biomedicine, industrialized countries are now faced with a large graying population; individuals in the "healthiest" countries now have a life expectancy at birth of 80 years and longer (Chapter 2). Although it is difficult to predict with certainty what will happen in the future, it is expected that this trend toward increased longevity will continue, at least in the most developed countries. For example, it is predicted that by the year 2030, about one-fifth (18%) of the population of the United States will live to 65 years and longer (4,5).
Today, in several developed countries, the rise in the proportion of the elderly relative to the total population is associated with a decline in the proportion of young people (Chapter 2). This phenomenon has to do not only with the increased longevity of the elderly, but also with the decrease in the number of the young persons. Despite a reduction in infant mortality worldwide, attributable to successful preventive public health and medical interventions, we have seen a concomitant reduction in fertility, which has been attributed to various socioeconomic and lifestyle factors. For example, in Italy, the average number of births in the family has fallen from 4.67 in 1850 to 3.14 in 1900, to 1.88 in 1950, and to 1.3 in 2005 (6,7). To adjust to a population shift where the elderly represent a predominant group, societies must rearrange their economic, technologic, medical, and educational priorities. Such adjustments are as costly as they are difficult to accomplish, for while both younger and older persons have a similar degree of vulnerability, their needs are quite different. Thus any change in priorities must be based on a solid understanding of the fundamental principles that regulate human aging if we are to achieve optimal social, economic, and medical support for both groups.
In regard to older persons, such planning necessarily involves (i) repudiating persistent myths and stereotypes of old age (Box 1) and replacing them with a firm grounding in what we now know about the physiology of human aging, and (ii) adopting new guidelines compatible with recent demographic changes (Box 2) and incorporating a realistic view of current and future physiotechnological advances. According to the renowned economist Fogel, "health care is the growth industry of the twenty-first century. It will promote economic growth through its demand for high-tech products, skilled personnel, and new technologies" (2).
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For centuries, ever since the legendary Ponce de Leon went searching for the elusive Fountain of Youth, people have been looking for ways to slow down the aging process. Medical science has made great strides in keeping people alive longer by preventing and curing disease, and helping people to live healthier lives. Average life expectancy keeps increasing, and most of us can look forward to the chance to live much longer lives than our ancestors.