When Apple releases a new version of iOS (OS stands for Operating System) many people rush to install it right away. This may not always be a good idea. If your device is not from the two most recent generations of iPhone then you may encounter some problems and slow-downs even though technically your device will be supported by the latest OS version. You get 3 years of being current with an Apple iPhone and only 2 years where you are likely to have excellent performance on the current OS.
If you choose not to upgrade you will be running a few risks. One is that the apps you run will no longer be updated for your platform, most developers stop doing fixes for apps on older versions of the OS. And many developers will start releasing the same app but it will require the newest OS to even install.
Conversely, these things won't happen right away. Many developers will wait a few months before making the jump completely to newest OS versions. Here's the sad truth with Technology. A device that's parts and technology are 3 years old (like an iPhone 4S today) is really slow compared to todays devices. Progress is always driving capabilities forward, and companies are always trying to drive people to buy the latest device. Today with the 4S, the iPad 2 and the iPad Mini we are looking at a device that is 3 years old, even if you buy it brand new today. Apple was kind enough to provide an iOS update for our devices. There are two philosophical approaches to this situation.
Go with the Flow
Everything Changes. Learn to adapt to the new stuff and just upgrade as soon as its convenient. Apple usually gives us a few performance optimizations in the *.1 release anyway. Just suck it up and look forward to when you can afford a new device. The pro column of this philosophy looks pretty good, you get lots of shiny new features! Some things become a lot easier (like extensions in iOS8) and you get to be part of the cool crowd that is using the latest and greatest software. You get to install and run all the latest cool apps that only work on the most recent OS. The con column contains mostly performance decreases, and the chance that you will get burned by some temporary bug that ships in the first version of the operating system.
or Stick to your Guns
Your device works fine like it is. Why risk a big change. If I don't absolutely need one of the new features, then I'll stick with the old version. This is the philosophy of getting a device to accomplish a few specific tasks and not worrying about the other possibilities that you may or may not ever use. If the old iOS gets the job done, why try to change. If your iPod Classic plays music for you just fine, why upgrade? If your digital camera takes adequate pictures for you, why lust after the latest and greatest? Do those feelings sound familiar to you? This could be your path forward.
My wife has the same iPhone 4S as me, that never upgraded to iOS7. She stayed on 6. She does email in the Gmail app, she sends and receives a lot of text and iMessages, and she makes and receives a few phone calls. She reads books in Kindle. Occasionally she tries to get directions from Maps. She doesn't play games hardly ever. In this case she stuck with the old versions of the apps, turned off automatic updates and is perfectly happy. Whenever I pick up her phone to use it I am amazed at how fast and responsive it is. It just feels so much faster than my phone on iOS7.