According to Apple’s App store measurement for the end of 2013 shows that 75% of their users have upgraded to iOS 7.
This leaves a questionable 22% behind on iOS6, and 4% of users on an earlier iOS (non-upgradeable).
Apple users have a higher rate of upgrading their operating system.
While compared to Android, only 54% of users use Jelly Bean. The remaining 46% are split between varieties of different Android versions.
This is not a downfall for android, as a lot of cheaper mobiles use older Android operating systems. This gives developers a larger audience to target on specific Android versions.