That’s not how that works. Broadly speaking, you compile your app against a selected SDK version. Want to use newer features? You have to use a newer SDK. The SDKs themselves support selected ranges of OSes and don’t go back all the way to the beginning.

If you’re writing an app that targets the newest hardware features, say because you’re making a camera app that uses the latest updates, it’s not going to run on iOS 5. You can’t hold that against app authors, or even against Apple, really. There’s not a lot of return on investment for sinking thousands of dev hours into supporting ancient phones that almost no one uses, and which by definition are more likely to be used by people who won’t spend a dime on apps or services.