Many native speakers like to use fractions to explain ratios in population or in other areas. You can
use either one depending on which you feel more comfortable using.
The expression "1 in 3" and "one third (1/3)" have the same meaning and can be used to express
the same idea.
多くのネイティブスピーカーは"fractions"（分数）を使って人口の割合やその他の事を説明するのが好きです。
どちらでも、使い易い方を使って表現することが出来ます。
この"1 in 3"（3つに1つ）という表現と "one third (1/3)" は同じ意味になるので、同じことを言いあわらすことが出来ます。
"one is three people are over 65." This explains that for every three people one is 65 or older.
"1 in 3 people are 65 or over." This also explains that one in three are older than 65 but uses numbers instead of letters.
"one of three people are over 65."
(3人に一人が65歳より上です。)
"1 in 3 people are 65 or over."
(3人に一人が65歳以上です。)
文字ではなく数字で表したバージョンです。
Amongst three people one of them is 65 years old or older
The ratio of people that are 65 years old and older is 1 as to 3
*Amongst three people one of them is 65 years old or older. - When comparing two or more things use older/oldest.
Example Sentences:
My brother is 5 years older than me.
Jack is 15 years old, Tim 17 years old and Jill is 13 years. Tim is the oldest one.
*The ratio of people that are 65 years old and older is 1 as to 3. - Ratio is a comparison between two or more things.
Example Sentences:
The ratio of boys to girls in our class is 1:2(1 as to 2).
When baking a cake, the ratio of flour to baking powder and butter should be proportional.
*Amongst three people one of them is 65 years old or older.
（3人に一人が65歳かそれ以上の年齢です）
- 2つの物やそれ以上の物を比べる時はolder/oldestを使います。
【例文】
My brother is 5 years older than me.
（私の兄は私に5歳年上です）
Jack is 15 years old, Tim 17 years old and Jill is 13 years. Tim is the oldest one.
（ ジャックは15歳、ティムは17歳、そしてジルは13歳です。ティムが一番年上です）
*The ratio of people that are 65 years old and older is 1 as to 3.
（65歳以上の人口の割合は3人に1人です）
- "Ratio"とは二つの物やそれ以上の物の割合の事です。
【例文】
The ratio of boys to girls in our class is 1:2(1 as to 2).
私たちのクラスの男子と女子の割合は1対2です。
When baking a cake, the ratio of flour to baking powder and butter should be proportional.
（ケーキを焼くとき小麦粉に対してベーキングパウダーと牛乳は比例しなくてはいけません）
Well, you may quote a statistic in any situation to highlight a point you wish to make. You could concentrate on the young population percentage as in the first example. Or, you could give details of the older part of the population as in the second sentence, depending on what is best for your purposes in any given scenario.
アメリカ合衆国 
南アフリカ 
インド 
日本 
イギリス 
