Although Apple is one of the best handset-makers around when it comes to supporting older devices, it does have a cutoff point. And for iOS 7, that cutoff point was the iPhone 4, meaning you can not install the new firmware on devices older than that.
But never fear! If you happen to be running an older device, you can still get the look and feel of iOS 7 thanks to Whited00r 7. The modded firmware includes an iOS 7-style UI with card multitasking, Control Center, push notifications, and much more...