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?