Family Sharing - https://support.apple.com/HT201060 - "You can use Family Sharing on your iPhone, iPad, or iPod touch with iOS 8 or later, your Mac with OS X Yosemite or later and iTunes 12, or your PC with iCloud for Windows."
is from an old version of that document and Apple no longer has system requirement information posted in that document. My copy probably dates from when anything with iOS8 still was considered viable in the Apple ecosystem. Note that Apple works on the basis that everything you own is running current software versions and if you have something that can't be updated then it is "unsupported" (= they don't want to hear if it doesn't work and will probably have no idea if it does or not because they probably only tested things with the last 3 or 4 OS versions anyway). In that case if it works you're lucky, and if it doesn't then what are you doing still using something that old??? (That's them, not me; I still have 2 iPad 2s ;-) )
Not a true answer to your question but I that's the way things are.