The iPhone 5s shipped with IOS7. 5 major OS updates ago and it is not only supported, but runs faster on the newest version IOS.
To put things into perspective, the flagship Android phones that were released the same year were the LG G2, Samsung Galaxy S4, HTC One (the original one, not the M8), Nexus 5, and the Galaxy Note 2.
There are current flagships from all of the companies above that don’t even have the latest version of Android.. Even the Nexus 5, a phone made by Google, stopped getting updates 3 versions of Android ago.