I would like to create a basic app that would work even on the very old iPads. Is it possible? Will Apple approve that ?
Can I create and submit an app that will be retro-compatible from iOS 5 to iOS 11?
Apple's approval shouldn't be the problem as long as you properly support current iOS devices as well (e.g. 64 bit support).
I believe that the bigger issue will be to actually build the app with Xcode. The oldest deployment target supported directly by Xcode 9 seems to be iOS 8.0. If you wish to target older version of iOS, modifications of the Xcode.app are apparently needed (see https://eion.robbmob.com/blog/2016/11/18/building-for-ios5-with-xcode-8/).
The old iPads have to be running iOS 5.1.1 or later, as only those iOS versions include support for apps containing 64-bit slices. And Apple now requires submitted apps to include 64-bit support. Apps built with earlier versions of Xcode run just fine on iOS 11 devices as long as they include 64-bit support (an arm64 slice). IIRC, Xcode 5.1 or 6.x can build apps compatible with both iOS 5.1.1 and 64-bit iOS 11 devices. You can have multiple versions of Xcode installed on your Mac (or you may need to use an older Mac running a older release of OS X), and use the newer releases of Xcode to install test apps built with older releases on newer devices.