And at another thread here people were saying how they add a fake hardware requirement (e.g front facing camera) just to deny people from installing in older phones because 'it run slow there'.
If you want to deny people from installing on older devices, you can just set the target to iOS 7, meaning you need iOS 7 to install it. Doing that means it can only be installed on iPhone 4 and later, iPad 2 and later, and iPod Touch 5th gen and later.