Ask the iPhone or iPad owners who couldn't update to the latest iOS because their device wasn't supported or the iPhone owners who didn't get Siri for some reason. Fragmentation.
iOS 6 runs on all iPhones 3GS and up. That phone came out in 2009. It also runs on the iPad 2 and up, plus the iPod Touch 4th generation and up.
Almost 90% of iOS devices out there are running iOS 6, according to
this developer.
Specific features like facetime and siri have absolutely no bearing on developers at all, since these features cannot be accessed by apps. However, the core functions and APIs are identical across the vast majority of iOS devices, since so many of them are running the latest major version. This is not fragmentation.
Android, on the other hand, is split pretty much 50/50 with half using some form of the latest major version and the rest using 2.3, 2.2, 3.0, etc.., so developing for Android is trickier.
An app developer would say: "I want to write an app for Android and iOS, which OS feature sets do I target to reach the maximum audience? Well, with Android, approximately half the users are using 2.3 and the other half are using 4.0, so I will develop my app using the 2.3 featureset so it can run on both 2.3 and 4.0, and leave the 4.0 featureset untouched until I can afford to abandon the sub 4.0 users.
My iOS app, however, will be coded for iOS 6, since the vast majority of iOS users have iOS 6."
So yes, iPhones and iPads have different features based on their age, but an app written for iOS 6 will work on an iPhone 3GS from 2009. Would an app written for Jelly Bean work on an Android phone released in 2009? (Hint: 2.0.1 Eclair came out in December 2009)