ご質問ありがとうございます。
全力で回答します。
｛英訳例｝
Japan's population is aging fast.
↓
Japan's population
[日本の人口](https://eikaiwa.dmm.com/uknow/questions/57996/)
age fast
急速に[高齢化](https://eikaiwa.dmm.com/uknow/questions/91060/)する
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｛解説｝
「日本は急速に高齢化している」という意味です。
「現在進行形」を使うとうまく言い表せると思います。
population は「人口」、
age は「年を取る」という意味です。
「高齢化する」は「population（人口）が age（年を取る）」と言います。
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｛例｝
China's population is aging fast.
⇒中国は急速に高齢化している。
Germany's population is aging fast.
⇒ドイツは急速に高齢化している。
The U.S. population is aging fast.
⇒米国は急速に高齢化している。
【出典：Motley Fool-Aug 26, 2016】
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お力になればと思います。
ありがとうございました。
The aging population of Japan is increasing at an alarming rate
The elderly population is booming
Booming = having a period of rapid growth.
"The booming economy of the 1920s."
An alarming rate = is one that makes you feel afraid or anxious that something unpleasant or dangerous might happen.
"The disease has spread at an alarming rate."
Boomingとは、急成長している期間のことを指します。
"The booming economy of the 1920s."：1920年代の経済成長期
An alarming rate とは、何か悪いことや危険なことが起こるかもしれないと怖いと感じる勢いや程度のことを指します。
"The disease has spread at an alarming rate." 恐るべき勢いで病気が蔓延している。
examples of use
"the social demographic of a countries ageing population"
"Japan has a high number of elderly citizens".
"The UK has a high volume of older (or elderly)
people in the population".
使用例
"the social demographic of a countries ageing population"
高齢化している国の社会人口学
"Japan has a high number of elderly citizens".
日本には多くの高齢者がいる
"The UK has a high volume of older (or elderly)
people in the population".
イギリスは、人口における高齢者の人数がかなり多い。
To express that Japan has a problem with the age of its population, use the sentences above.
When a country has a negative birth rate, it means that the population has a higher number of older people than younger people, not many births take place to equal or balance the number of deaths taking place.
This is a problem because if the number of youths is outnumbered by the elderly then certain processes in society will fail, certain systems and job positions will be hard to fill.
Negative birth rate: negative or zero natural population growth means that these countries have more deaths than births or an even number of deaths and births.
"In Japan, the number of elderly citizens is much higher than that of the youth."
The ageing issue in Japan is growing increassinly faster.
Japan's ageing population is an increasing problem.
When you want to explain that the ageing problem in Japan is growing faster, then you can say one of these two sentences.
For example.
How is life in Japan?
It is good, mostly.
Why mostly?
Well, Japan's ageing population is an increasing problem.
I hope this helps.
Have a great day.
Will
日本 
イギリス 
南アフリカ 
