Population ageing is the outcome of successful reductions in fertility, gains in infant and child survival and more recent gains in life expectancy at older ages. Fertility rates have declined dramatically across the world (see Fig. 1.2). At least 50% the world’s population live in a region in which fertility rates are below replacement level of 2.1 children per adult woman [4]. In India, fertility rates have fallen from 5.4 children per woman in 1970–1975 to 2.5 children per woman in 2015.

In association with declines in fertility and corresponding to better child survival, human life expectancy has also increased dramatically over the twentieth century [5]. More recent gains in life expectancy have been attributed to better survival in adult life, with increases in life expectancy at age 60 and at age 80 years [6]. These increases in life expectancy at older ages further change the structure of the oldest age groups.

In some populations, international migration has also had a role in changing age (and gender) distributions, but generally migration has had less of an impact than reductions in fertility and mortality rates [7].

Population ageing and increases in life expectancy are accompanied by changes in patterns of prevalent and incident diseases. As populations age, countries undergo major transitions in the main causes of illness and burden of disease, with increasing burden of non-communicable disease. According to the Global Burden of Disease Study 2013, almost 65% of deaths are due to non-communicable disease. Many of these diseases are most common at older ages. However, not all of the change in disease profile is due to ageing. Many countries are also experiencing massive economic development and increasing urbanization, along with substantial changes to lifestyle and increasing prevalence of risk factors including unhealthy diet, physical inactivity, obesity, hypertension and tobacco use [14]. Such epidemiological trends have been observed in India, China, Latin America and in parts of Africa [15]. For instance, it is estimated that nearly 15% of the global burden of diabetes is accounted for by 35 million people with diabetes in India [16]. Where people live in poverty, the onset of non-communicable diseases begins to occur at earlier ages with a pattern of “post-transitional illnesses in pre-transitional circumstances”. For these people, chronic disease will dominate their adult lives with likely very high rates of disability at older ages [17]. In addition to the rise of non-communicable disease, many rapidly ageing countries continue to experience high levels of infectious diseases including childhood infections, HIV/AIDS and high infant and maternal mortality rates [15].

Table 1.3 shows the most common causes of death [18]. Globally, the most common causes of death are cardiovascular diseases, cancers and chronic obstructive pulmonary disease.