Apple Begins Offering 'Last Compatible' Version of Apps for Users Running Legacy Versions of iOS [macrumors.com]

[…] Specifically, when a user tries to download an app that is not compatible with his or her version of iOS from the App Store , Apple offers the user the option to download an older version of the app that is compatible with the device.

Der App-Anbieter kann aber entscheiden [tekrevue.com], ob alte Versionen seiner Programme zur Verfügung gestellt werden, deswegen funktioniert das nicht bei jeder App.

If you do not want previous versions of your app to be available, for example due to a usability or legal issue, you can manage their availability in the Rights and Pricing section of the Manage Your Apps module on iTunes Connect.