What is the minimum ios version App store support?
The current App Distribution Guide says "For example, the lowest available setting for iPad apps is iOS 4.3." ( Setting the Deployment Target ) but my Xcode 7.1.1 doesn't seem to allow any deployment target for an iPhone project to be lower than iOS 6.0. So YMMV I guess. Basically the minimum is whatever the minimum supported Xcode version (Is Xcode 5 still allowed or does the store now require Xcode 6? I don't know) allows in the popup for the project deployment target.
One other factor - if you use Swift that limits you to iOS 7.0 and later.