I agree with the accepted answer, but would like to add some info based on experience I just went through with a client project.
The project is iOS 5.1+ compatible, but looks terrible when rendered with iOS 7 UIKit controls (buttons, table views, etc). The client isn't ready to to change the graphics to make the rendering of those iOS 7 visual elements not clash badly.
So - Have to be able to test and release a version of the app that is build against iOS 6 interface/lib (the iOS 6.1 SDK).
Using instructions here I was able to get the iOS 6 SDK installed in my Xcode 5:
Is it possible to install iOS 6 SDK on Xcode 5?
I then changed my project settings to build with iOS 6.1 SDK.
That allows me to run on my iOS 7 device (for testing) and shows all the UI elements as they looked in iOS 6.
Note: I did not go to the work of getting the iOS 6.1 simulator SDK for testing. You may want to do this. (I have network/other issues that mandate all testing really has to be done on a real, configured device.)
End result: I have iOS 6.1 SDK available to link my project against, and the effect of this at runtime is that the app calls the iOS 6.1 support library in iOS even though the device is running iOS 7 and has the iOS 7 support library available.
See also here:
http://blog.spacemanlabs.com/2013/09/how-to-support-old-ios-sdks-in-xcode-5/