Right now running on iOS 8.1.3, most of the existing iOS devices are capable of running the upcoming iOS version. For future projections, until fall where we will have iOS 9, there will be software updates regularly. Now its 8.1.3, tomorrow it will be iOS 8.2 followed with iOS 8.3 and future rumored iOS 8.4.
Unlike Android, iOS is proprietary of Apple products only. This makes pretty clear statement that whenever there is a software update to iPhone 6, it will be available to other eligible Apple devices the same day. For example the iOS 8.1.3 was made available on the same day to iPhone 6, 6 Plus, 5S, 5C, iPhone 5, 4S, iPod, iPad Mini, iPad Mini 3, iPad Mini 2, iPad Air, iPad Air 2.
One thing should be said that if 8.3 and 8.4 becomes official, it will be for the first time in history that Apple has opted for .3 release of any iOS version and most surprisingly the .4 version as well.
Currently there is iOS 8.1.3 update available for all eligible iDevices and soon iOS 8.2 will be released. Do checkout for iOS updates as it happens on Tec Advices.