Apple is reporting that almost three-quarters of their user base are now running on iOS 7. iOS 7 launched in September, which means that the adoption in 3 months was rapid. This chart was posted on the Apple developer supper center website:
The data is based on the devices that connect to the Apple App Store.
Android has a slower adoption rate to the new operating system due to fragmentation across many devices. There is currently only 1.1% of Android devices that use Android KitKat. KitKat is compatible with only a small number of devices. Android Gingerbread is three years old and is on around 24.1% of devices.