Not necessarily. Surely, some features, introduced with latest iOS versions, may not be supported or function, but you can download the app nonetheless. Concerning current app versions, Kik offers support for iOS 7 and newer versions, while Snapchat and Instagram instead need iOS 8 to be installed. If your device is on an older OS, you may still download an older version of the app (if, before being updated, they had an older version supporting your current OS), you just need to have them in your purchase list (if you don't, as they are free apps nonetheless, just open iTunes on a PC, use the same account you use on your device to login an do download them). If you tap on the cloud icon next to the app name, you will be prompted to download an older vesion instead of the newest (the last version that supported your iOS)