日本の人口は子供より高齢者のほうが多いって英語でなんて言うの？

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.

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

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. "

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.

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

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.

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

• 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.

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.