Apple today released a short memo to developers titled ‘Make Your Apps Work Seamlessly with iOS 7′, putting in place a few strict guidelines for apps and app updates. The new guidelines start on February 1st and required developers build their apps using the latest version of Xcode 5. In addition, all apps must be optimized for iOS 7. Apple points developers to the rather in-depth iOS Human Interface Guidelines for iOS 7.
This is a sunsetting of sorts for iOS 6 development, which makes sense. iOS 7 has been out for almost three months. Developers have had access to the SDK since June. There are still a large number of apps that haven't been updated to match the design aesthetic associated with iOS 7. While it's understandable that it takes time for developers to perfect their updates, Apple has provided a sufficient window for developers to get up to speed. This requirement doesn't affect apps that are already in the App Store, only new apps or updates.
The iOS 7 adoption rate is high, with over 75 percent of iOS users running the latest operating system. That number is likely going to increase over the holidays, with an influx of new iPhones and iPads. Older apps designed for iOS 6 look increasing more out of place. This move by Apple should help gently push developers in the right direction and that's building for iOS 7.
Source: Apple Developer Portal