5人に1人が75歳以上って英語でなんて言うの？

"One is five people are 75 or older." and "1 in 5 people are over 75." are both short ways of expressing that in every five people one is 75 or older.

In every five people, one is 75 years old or above. - This means that it is possible that in a group of 5 people amongst them one is 75 years.
The ratio of people who are 75 years old and above is 1:5- Ratio means proportion. It is usually used to refer to probability.
1:5 read as one is to five.

You may either describe the number of older people whose age is past the threshold of 75 as being 'twenty percent,' or, as a statistic of age distribution for a country, you could refer to the 'eighty percent' of people who are under 75.
"My grandfather is one of the twenty percent of pensioners aged over 75."

If you are talking about statistics then you can say 'every 1 in 5' means one person out of 5 people or a fifth (1/5)

These sentences give different examples of how you can say '1 in 5'. It is common in English to just say an age as '75' or whatever the number is, without adding 'years' or 'years old'.
The terms 'over', 'or older' and 'or over' all describe that the age includes 75 years and then goes above this as well. Therefore, they are not just 75 and they are not younger than 75.

One in five' or 'one-fifth' is the correct way to say this amount. It is also important to clarify which population you are referring to unless it is already very obvious from the previous conversation.