Xcode - Run Simulator with older iOS Version

Please note that matt's solution is not available on new version of OS X (El Capitan). Here are all the available Simulators:

In the Downloads pane of Xcode's Preferences, download the iOS 7 or 7.1 simulator.

After it is downloaded, go to the Devices window (Window -> Devices) and click the Plus button to make a new simulated device with the system version you want. For example you might decide to make an iPhone 4s that's running iOS 7.

Now you'll be able to choose that device/system as the destination when you build and run.