Population ageing(Aging Population also) or population aging (see English spelling differences) occurs when the median age of a country or region rises. With the exception of 18 countries termed by the United Nations 'demographic outliers' (see the Ud 2005 Human Development Report) this process is taking place in every country and region across the globe.
Population ageing is a highly generalized process. It is most advanced in the most highly developed countries. Among the countries currently classified by the United Nations as more developed (with a population of 1.2 billion in 2005), the median age of the population rose from 29.0 in 1950 to 37.3 in 2000, and is forecast to rise to 45.5 by 2050. The corresponding figures for the world as a whole are 23.9 for 1950, 26.8 for 2000, and 37.8 for 2050. In Japan, one of the fastest ageing countries in the world, in 1950 there were 9.3 people under 20 for every person over 65. By 2025 this ratio is forecast to be 0.59 people under 20 for every person older than 65.
The sources of population ageing lie in two (possibly related) demographic phenomena: rising life expectancy and declining fertility. An increase in longevity raises the average age of the population by raising the number of years that each person is old relative to number of years in which he is young. A decline in fertility increases the average age of the population by changing the balance of people born recently (the young) to people born further in the past (the old). Of these two forces, it is declining fertility that is the dominant contributor to population ageing in the world today.
More specifically, it is the large decline in the total fertility rate over the last half century that is primarily responsible for the population ageing that is taking place in the world’s most developed countries. Because many developing countries are going through faster fertility transitions, they will experience even faster population ageing than the currently developed countries in the future.
Canada has the highest per capita immigration rate in the world, based largely on the rationale of countering population ageing. The C. D. Howe Institute, a conservative think tank, has suggested that immigration can not be used a viable means of countering population ageing. This conclusion is also seen in the work of other scholars. Demographers Professor Peter McDonald and Dr Rebecca Kippen comment, "[a]s fertility sinks further below replacement level, increasingly higher levels of annual net migration will be required to maintain a target of even zero population growth."
Most of the developed world (with the notable exception of the United States) now has sub-replacement fertility levels, and population growth now depends largely on immigration together with population momentum which arises from previous large generations now enjoying longer life expectancy.
However population ageing also increases some categories of expenditure, including some met from public finances. The largest area of expenditure in many countries is now health care, whose cost is likely to increase dramatically as the population ages. This would present governments with hard choices between higher taxes, including a possible reweighting of tax from earnings to consumption, and a reduced government role in providing health care.
The second largest expenditure of most governments is education and these expenses will tend to fall with an ageing population, especially as fewer young people would probably continue into tertiary education as they would be in demand as part of the work force.
Social security systems have also begun to experience problems. Earlier defined benefit pension systems are experiencing sustainability problems due to the increased longevity. The extension of the pension period was not paired with an extension of the active labor period or a rise in pension contributions, resulting in a decline of replacement ratios. In recent years, many countries adopted policies to strengthen the financial sustainability of pension systems, although the challenges regarding pension adequacy remain.