The Japanese population is made up of more senior citizens than children.
まず一つ目の例ですが、
There are more old people in Japan than children.
（日本には子供より高齢者の方が多い。）
ここでは単純に There are more を使っています。There are more __ than __ ＝ 〜よりも〜の方が多い。という意味です。
二つ目の例：
The Japanese population is made up of more senior citizens than children.
（日本の人口は子供より高齢者で構成されている）
made up of ＝ 〜で出来上がっている、構成されている というような意味です。
日本の人口＝Japanese population, population of Japan とどちらでも大丈夫です。
高齢者＝ old people (カジュアルな言い方）、senior citizens
どうぞご参考に。
The ratio of elderly people to children in Japan is very high.
There are fewer children in Japan than elderly people.
There are more elderly people than children in Japan.
The word "ratio" refers to the size, quantity or volume of something relative to another thing.When expressed as a number, it is usually reduced to the least common multiple. For example, if you have 20 apples and 10 oranges, there is a 2:1 ratio of apples to oranges.
"ratio" は、大きさや数量の「割合」をいいます。数字で表す場合には普通その最小公倍数が使われます。例えばリンゴが20個、オレンジが10個あるなら、"There is a 2:1 ratio of apples to oranges"（リンゴとオレンジの比率は2対1です）。
The population in Japan is an ageing one, with elderly people outnumbering children
The proportion of elderly people in Japan is increasing, compared to children.
Compared to elderly people, children make up a minority of the population in Japan.
The first phrase highlights the fact that the Japanese population is ageing, and makes a comparison with the number of children being born.
The second phrase uses the word "proportion", and also compares what is happening with both groups.
The third phrase focuses on what is happening with children, and that they are becoming a minority group in population terms
We have a higher percent of elderly people in comparison to children.
The population of elderly people outweigh the amount of children.
The majority of the Japanese population are elderly people.
All of the above sentences describe how there are more elderly people compared to children.
Outweigh is a verb that means - to exceed in weight, value, or importance.
Majority is a noun that we use to refer to a higher number of something, more than half. "The majority of the Japanese population are elderly citizens. "
これらは全て、「子どもの数よりも高齢者の数の方が多い」と伝える言い方です。
"Outweigh" は「〔重さ・価値・重要度が〕～を上回る」という意味の動詞です。
"Majority" は「大多数／過半数」という意味の名詞です。
"The majority of the Japanese population are elderly citizens. "
（日本の人口の半分以上は高齢者です）
1. Japan has an aging population.
An aging population is one in where there are many
elderly people and too few children.This problem was
brought about by the high life expectancy and very low
fertility rates in Japan.
2.In Japan, the elderly outnumber the children.
The elderly are senior citizens over the age of 65 years.
In Japan they outnumber, or are more numerous than
the children. The elderly outnumber the children by almost
three to one.
1. Japan has an aging population.（日本は人口が高齢化しています）
"An aging population" は、高齢者が多く子どもが少ない状態をいいます。日本では、平均寿命が伸びたことと出生率の低さがこの状態を引き起こしています。
2. In Japan, the elderly outnumber the children.（日本は子どもの数より高齢者の数の方が多い）
"The elderly" は65歳以上の高齢者を指します。日本では、"The elderly"（高齢者）の割合は子どもの3倍近くになっています。
In Japan, there are more elderly people that children.
There are considerably fewer children than elderly people in Japan.
In Japan, there is a large aging population.
The first two sentences express the difference in age population with comparative structures, 'more' ...... 'than' or 'fewer' ..... 'than'. 'Fewer' is used with plural countable nouns and 'less' is used with uncountable nouns.
The third sentence refers to the size of Japan's aging population but doesn't compare it to the younger generation
There are more elderly people in Japan than children.
In Japan, the elderly outnumber the children
The number of children in Japan is not as great as the number of elderly people
When talking about the population of Japan, you want to say that there are more elderly people than children. Any of the above suggestions is adequate for that scenario.
In Japan, there is a large aging problem when there are more old than young people
Your example is absolutely fine but I provided you with one more option. The differences are that it stresses that it's a problem. For example - Japan is facing a big aging problem. There are more old people than children
あなたの考えた例でも全く問題ありませんが、もう一つ別の言い方をご紹介しています。違いは、二つ目の例ではこれが問題であることを強調しています。
例：
Japan is facing a big aging problem. There are more old people than children（日本は高齢化に直面しています。子どもの数よりも高齢者の数の方が多くなっています）
The majority of the population in Japan are elderly people.
- Within the first example we are using the typical, "more...than," style of comparison when talking about two different things in relation to each other.
- Within the second example we use the word, "majority," which describes, "the highest percentage," to describe that there is a high amount of one particular thing.
Japan's population consists of more elderly people than children.
In Japan, there are more elderly citizens than there are children.
Japan's population consists of more elderly people than children.
A "population" of a country refers to all of the people who are citizens of that particular country.
"Consists" means what something is made of or what it is composed of.
In this answer, you are then saying that the population of Japan is made up of more elderly people than children.
In Japan, there are more elderly citizens than there are children.
A "citizen" is someone who legally lives in a country or legally apart of a particular country's population. Citizenship can be because of being born in that country or because someone has applied to become a citizen of that country.
In this answer, you are saying that the amount of elderly citizens of Japan is more than the amount children citizens.
Japan's population consists of more elderly people than children.（日本の人口は子どもよりも高齢者の方が多い）
"population" はその国の全ての国民を指します。
"consists" は「～で構成されている／成り立っている」という意味です。
この例では「日本の人口は子どもよりも高齢者の方が多い」と言っています。
In Japan, there are more elderly citizens than there are children.（日本は子どもの数よりも高齢者の数の方が多い）
"citizen" はその国の法律上の国民を指します。"citizenship"（国籍）は、出生または帰化によって取得されます。
この例では「日本は子どもの数よりも高齢者の数の方が多い」と言っています。
日本 
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イギリス 
オーストラリア 
南アフリカ 
