Apple has given developers a deadline to make all app submissions optimized for iOS 7. The deadline for all submitted apps to be optimized for the new OS is February 1, 2014. The optimization requirement holds true on all new apps as well as updates to existing apps.
The optimization that Apple wants will require all apps to be built using the latest version of Xcode 5 including 64-bit support and access to new features in the OS like Background APIs. Apple has previously stated that the majority of iOS users are running on iOS7.
The last time Apple offered adoption rates for iOS 7, 76% of all iOS users were on the latest version. Apple has noted that developers can optimize their apps for iOS 7 and build it with the new Xcode without having to completely redesign the app.
Exactly what that optimization involves is unknown. Reports indicate that with some of the standard UI elements changing in the new SDK, some apps will need work to look and function correctly in iOS 7. Check out our review of iOS 7 for more details on Apple's latest operating system.