ご質問ありがとうございます。
一例をご紹介します。
《解説》
日本語の「少子高齢化」のように決まった言い方はないと思います。
メディアでは日本の「少子高齢化」は以下のように表現されています。
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《例》
Japan is facing a declining birth rate and an aging population.
日本は少子高齢化に[直面](https://eikaiwa.dmm.com/uknow/questions/86719/)している。
【出典：The Japan Times-Apr 5, 2012】
Population is a central problem confronting Japan. A falling birth rate and an aging population mean that the country has far too few young, productive workers.
[人口](https://eikaiwa.dmm.com/uknow/questions/36979/)問題は日本が直面する主要課題の一つ。日本は少子高齢化に直面し、若い生産性の高い労働者が圧倒的に不足している。
【出典：Wall Street Journal-Jun 14, 2011】
Japan needs to expand its work force, which is shrinking rapidly as a result of a sagging birth rate and an aging population.
日本は、少子高齢化により急速に縮小する労働人口を増加させる必要がある。
【出典：New York Times-Aug 22, 2013】
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国によっては birth rates と複数形が使われることもあります。日本の「[少子化](https://eikaiwa.dmm.com/uknow/questions/26405/)」には birth rate（単数形）が使われることが多いです。
お役に立てば幸いです。
ありがとうございました。
An age distribution problem which primarily causes economic imbalance of workers and people of retirement age
In general population aging is a worldwide issue affecting developed nations at the present time. This scenario puts increasing pressure on the younger working population to provide retirement income and facilities such as hospitals for old folk.
You could say: "It's a country full of pensioners!" as a joke.
一般的に[高齢化](https://eikaiwa.dmm.com/uknow/questions/91060/)は世界中の[先進国](https://eikaiwa.dmm.com/uknow/questions/55848/)のなかで問題になっています。これは[年金](https://eikaiwa.dmm.com/uknow/questions/43210/)問題や社会保障費などから若い労働世代に大きな負担となっていますよね。冗談っぽくこういえます。
"It's a country full of pensioners!"：老人ばかりの国だよ！
eg. Japan's population is projected to decline rapidly over the next few years due to the aging population and declining rate of births.
例えば、以下のように言えます。
Japan's population is projected to decline rapidly over the next few years due to the aging population and declining rate of births.
日本の人口は、出生率の低下と人口の高齢化により数年で、急速に減少るると言われています。
Ageing' generally refers to the process of growing older, but in this case does refer to the older population.
More generally, one can also describe the dynamic (dynamic = changeable variable) of the population change as being 'mismatched'. Ideally a matching trend would show an equal death and birth rate.
Population decline can be described as a situation where the population is aging and the birthrate is declining. This means that there are more retirees than workers and children. A declining birthrate reduces the number of young people. In the long term, the whole population will have declined because despite any effect on their life spans, the aged will naturally die, leaving a smaller young population.
So, you may say:
Our country is facing a long term population decline because of an aging population and a declining birthrate.
or
We have a declining population because of an aging population and a declining birthrate.
"Population decline" は、人口が高齢化し、出生率が低下している状況を表します。労働者や子どもたちよりも退職した人のほうが多い状況です。
「declining birthrate（出生率の低下）」は、若い人の数を減らします。寿命の変化に関係なく、年を取れば人は亡くなります。減少する若者が残されるため、長期的には総人口が減ることになります。
以下のように言えます：
Our country is facing a long term population decline because of an aging population and a declining birthrate.
〔訳〕私たちの国は少子高齢化によって長期にわたる人口減少に直面しています
または、
We have a declining population because of an aging population and a declining birthrate.
〔訳〕私たちは少子高齢化によって人口減少に直面しています
A country with a lot of the older generation and not enough young.
The country has a slow birth rate but a high ageing population
A population is the number of people within the country no matter what gender, age etc. When we say high ageing population, this means that there is a high number of people who are a lot older than younger. With not so many people having babies, the birth rate is slow and small and then this isnt enough to replace the older generation.
日本語の「少子高齢化」が英語で「an ageing society with a low birthrate」といいます。
以下は例文です。
日本は少子化や高齢化が問題になっています ー Japan has a big problem with an ageing society with a low birthrate
日本は高齢化社会と低出生率の結果として外国人労働者の雇用を余儀なくされています ー Japan is being forced to hire foreign workers as a result of the ageing society and low birth rate
参考になれば嬉しいです。
the number of elderly people is significantly higher compared to the amount of births per annum
There is no single phrase that can fully capture the expression " an aging population and a declining birthrate" and so we use it in its extended form.The simplest way would be to say :
-the number of elderly people is significantly higher compared to the amount of births per annum(per annum means per year.)
ワンフレーズで「少子高齢化（an aging population and a declining birthrate）」を完璧に表すことはできません。少し長い説明が必要です。
シンプル例としては：
the number of elderly people is significantly higher compared to the amount of births per annum (per annum means per year.)
〔訳〕
年間の出生率が低いのに高齢者の数がすごく多い（"per annum" は「per year（1年につき）」と同じ意味）
A country with an aging population but low birth rate.
If you are wondering what you can cal a country with an aging population but decreasing birth rate, you can say something like "A declining birth rate with an aging population.". This means that most people in this specific place are old and that not man babies are being born either therefore causing the country a birth rate population problem.
Older population refers to the people in that particular town, city, district or country being of older age.
Low birthrate refers to the fact that there are not many babies that are born in that particular town, city, district or country.
We would use the expression, "an aging population," to describe that there are a lot of people who are much older and usually at the ages where people usually retire.
"A declining birthrate," describes that there are less people who are getting married and having children to replace the new workforce as people continue to age.
Declining birthrate and increasing elderly population
Less children and increasingly more elderly people
少子高齢化のことは「Declining birthrate and increasing elderly population」と言います。
例文:
- The declining birthrate and increasing elderly population is a growing issue in Japan.
(少子高齢化は加速しつつある日本の社会問題だ)
少子高齢化のもっとカジュアルな言い方は「Less children and increasingly more elderly people」。
例文:
- Japan has a problem with less children and increasingly more elderly people.
(日本は少子高齢化という問題を抱えている)
ご利用いただいきありがとうございます。
またの質問をお待ちしております！
the best way to describe this would be to say "here in japan we have an aging population and a declining birthrate." this will explain that majority of the people in your country are getting older and that your country is having less children than normal.
These phrases here are explaining that the current population is growing older and older while the current rate of children being born is decllining / slowing down.
there is no term for this, so the best way to describe this is to explain it within the sentence.
日本 
イギリス 
カナダ 
南アフリカ 
オーストラリア 
アメリカ合衆国 
ニュージーランド 
