Apple recently published statistics documenting a high rate of user adoption for the latest version 9 of iOS. As long as equipment and software environments are compatible, users often benefit with both improved security and functionality by staying on latest editions of operating systems. 

http://www.pcmag.com/news/344504/ios-9-adoption-rate-tops-80-percent

Apple’s iOS 9, which achieved the fastest iOS adoption rate ever in the days following its September 2015 release, is now installed on 84 percent of Apple’s mobile devices as of May 9, according to Apple’s developer website. Adoption of iOS 8 is at 11 percent, while earlier OSes are running on 5 percent of devices.

Users have long been keen to keep their iPhones and iPads up to date, but crossing the 80 percent mark for a single OS version is an important step as Apple gears up to release iOS 10. The company is expected to pull back the curtain on iOS 10 at its Worldwide Developers Conference next month and make it available in September.

Apple iOS 9 adoption numbers are also an indication of how quickly users are upgrading their devices. The operating system is compatible with phones as old as the iPhone 4s, as well as third-generation and newer iPads and fifth-generation or newer iPod touches. It’s also an improvement upon the adoption trajectory of its predecessor, iOS 8. Numerous bugs slowed iOS 8’s initial adoption, though it eventually reached 85 percent of Apple devices.