part 1 Aging population
The world's population is growing at a rate of a little more than 1% per year.
However, not all segments of the population are growing at the same rate.
This graph shows the rate of growth of 3 different groups: children, adults, and the elderly.
As you can see, while the population of children is fairly constant, the elderly population is increasing at the fastest rate.
This is due to very low birth rates in developed countries and birth rate declines in most developing countries.
As a result of these trends, the world's population is aging .
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Between 2015 and 2050 , the percentage of world's population over 60 is expected to nearly double.
In 2015 there were 900 million people over 60; in 2050 the number should be around 2 billion.
Aging populations are becoming a huge problem in many countries, especially developing countries.
It used to be that many elderly people lived with their family and helped take care of the next generation.
They had a place to stay and people to help take care of them when they needed assistance.
Now, however, many elderly people, or senior citizens, are left on their own.
This is partly the result of people having fewer children and also because of massive urbanization（n. 都市化；文雅化 urbanisation）.
Young and middle-aged adults have left the rural areas for economic opportunities only available in large urban centers.
As a result, there is no one to care for the elderly who are left behind to live on their own.
This is especially ture in developing countries.
( Different segements of the population are growing at different rates .)
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With an aging population, health problems become a growing concern.
Some of the leading conditions that accompany aging include vision problems, hearing loss, diabetes（糖尿病，多尿症）, and dementias（痴呆） such as Alzheimer's disease （阿耳滋海默氏病 / 老年痴呆症）.
These problems can lead to disabilities that make it difficult to live without assistance.
With diseases such as Alzheimer's, people lose their memory and are unable to recognize their own children.
Worldwide, more than 46% of people aged 60 and over have disabilities.
Getting meals and medical care, for example, becomes both expensive and difficult.
As a result, the costs of providing services to the sick and disabled can become a huge problem.
( Memory loss is one example of a disability.
With an aging population, there will be more sick and disabled people to care for.)
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part 2 Dependency Ratio
In economics , the dependency radio shows the relationship beteewn the number of people not in the labor force and those in the labor force .
Those not in the labor force are the dependent part of the population.
Those in the labor force are the productive part of the population.
A high dependency ratio means that there are fewer working people to support health, social security and education services, which are used by the dependent sectors of the population.
These number is caculated by adding together the total number of young and old people and diving that number by the number of working age people.
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Sometimes the dependency ratio is presented in two parts.
One part focuses on the radio between children and the working age population.
This is the dependency ratio for the young.
The other is the ratio between the elderly and the working age population, which is the dependency ratio for the old.
Here are some dependency ratios for the old in 5 countries: China, India, Japan, the US, and the UK.
It shows the ratios at 3 different points of time: 2000, 2015, and 2050.
Note that the greatest percentage change from 2015 to 2050 is for China.
The dependency ratio nearly triples, from 13.1 to 39.
The other countries show gains, but as a percentage increase, they are less.
In Japan the ratio increases from 43.6 to 71.8 , which is less than double.
（The dependency radio shows the relationship beteewn the dependent and productice parts of a population.）
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The life expectancy for Japan in 2050 is predicted to be 93, which is the highest of these countries.
A high life expectancy obviously increases the dependency ratio.
And note that the dependency ratio ignores the fact that those counted in the elderly segment of a population are not necessarily dependent.
An increasing proportion of them are working, and many of those in the working-age segment may not be working.
So this way of calculating the dependency ratio in a country can be misleading.
By pointing this out, we can see the danger of using such numbers to make policy without understanding how they are calculated.
In the end, details are important.
（The higher the life expantancy，the higher the dependency ratio.
When using numbers like dependency ratios, one needs to understand how they are calculated.）