iOS 6: 93% adoption according to App Store stats
Fragmentation is the Achilles' heel of operating systems. Owners continue to use older versions out of lethargy or perceived difficulties in upgrading. It is plaguing Android in the mobile field and has long been a problem with Windows where Microsoft has to be content with low single-digit uptake of new versions. This results in all sorts of backward-compatibility problems.
In stark contrast, Apple is sitting pretty. Only one percent of iOS users are still using iOS 4 and older while just six percent are stuck on iOS 5. The current version, 6, has been adopted by a remarkable 93 percent of users. The figures are compiled from App Store research so I suppose it is possible that there are customers out there who never buy apps and who could still be limping along on old iOS versions.
Nevertheless, this is an impressive statistic and underlines the unique position of the Apple eco-system.
The company is often criticised for nannying customers: Apple knows best. But because Apple keeps a strict control on both software and hardware, upgrades are made easy and are virtually automatic. Contrast this with the situation in the Android world. Even with OSX, upgrades are very cheap (compared with Windows, for instance) and there is every incentive to move to the latest version.
In the end we all benefit. There is less pressure on both Apple and developers to continue with backward compatibility and, as a result, we get a more stable environment in which to work.