I mean the people whose age are 40 to 49. Do I have to use "the people at their forties" or "the people in their forties"? I googled the "in their forties" and found this. In the example, there's a sentence "They stopped smoking in their forties." I also tried "at their forties" and could find sentences that use the expression.
Which one is right? If both are right, what are the differences between them?
Best Answer
For your example, the correct one uses the preposition "in": "… the people in their forties…".
I cannot conceive of a situation where, in the sentence examples you gave the word "at" could be used correctly.
One of the definitions of in is in the form of a preposition denoting a period or span of time. The word at also comes in the form of a preposition and may also indicate time. However it is specifically to a point in a span of time. For example you can use your example like this:
This denotes people who fall in a span of time- between the ages of 40 and 49.
This sentence has to use at because 49 is a specific point in time.
American usage may sometimes have in and at used interchangeably. However, you should always be mindful of the point of view, the tense, and the literal object to which the proposition refers.