The launch of iOS 9 was preceded by the announcement of Apple, at the developer conference in which it was presented by iOS 9, in which it stated that one of the priorities of that latest version of iOS, would focus on improving the performance of older devices, that after the arrival of iOS 8, the older models had become bricks.
With iOS 8, both the iPhone 4s and iPad 2 have become useless devices, with which you could barely perform tasks. But in the successive updates the performance of these two devices was improving a lot, until the final version of iOS 9 arrived and declined again, although subsequent updates have improved the fluidity of both the iPad 2 and the iPhone 4s.
Apple has posted on its website a section called Questions and Answers, in which it tries to solve the most common doubts that both future and current users may have. As we can see in the question related to the useful life of its devices, we see how according to the company it states that in order to continue enjoying all the features that the company adds each year, our iPhone must have a maximum of 3 years of life.