Does Apple have a policy regarding how long they will continue to release and support security fixes for each iOS release?
I observe that many people complain about performance issues with iOS upgrades on their relatively "old" Apple devices. Therefore, before buying, I would like to foresee how long my device will be provided with security updates by Apple for its default iOS version.
I specifically wonder the date of end of security support for iOS 6 (its latest sub-version, probably 6.1.3).
Edit in 2019: I have came across a graphic showing Apple's support for older devices. This is not specifically about security updates, but still I think it is relevant and interesting.