iOS version distribution of my own app
In iTunes Connect Analytics, select your app and go to Metrics → Active Devices or Sessions and choose View By → Platform Version. This will give you a breakdown of the iOS version installed per the category you selected.

Keep in mind this data only began reporting in iOS 8 and is opt-in only. You can hover the "Opt In Only" text to view the percentage of your users who have opted in which should provide a little guidance in extrapolation.