Recently I asked a friend to try Play The Field, after which he replied he could not because the game required iOS 8.3, but his iPhone was currently still at 8.1. I hadn’t realized the app was set such that “Deployment Target” (in the XCode project’s General settings under “Deployment Info”) was set to 8.3, which means that 8.3 is the minimum OS required to install the game. This was very valuable information, and in the next release I’ll change this to 8.0.
Just be careful because your program may be using Apple APIs which aren’t supported by certain older OS versions. The father back you go the more you have to worry about writing code to dynamically detect the version and use alternate APIs.
If you are new to mobile development you might take the stance of “Why don’t the users just upgrade to the latest OS?”, but that naive stance won’t help you get any more downloads. There are several reasons people might avoid an upgrade: risk avoidance of new bugs and security problems, inability to upgrade on a jailbroken device, and fear of bricking their device. You may think having a device destroyed by an upgrade is impossible, but I have had a MacBook Pro totally messed by a failed OS upgrade before.
I doubt that once I remove this restriction the downloads will come pouring in, but it’s literally a single click to fix (excluding the whole process of uploading a new version to the app store, which I would have to do eventually anyway). Creating an iPhone version was significantly more difficult, but had the same end result – reducing a reason that user’s couldn’t download the app.