[sjwright]

Which implicitly means that swift code can be compiled for iOS 7. Which isn't surprising since the compiled code uses the ObjC runtime.

[chucknelson]

When they talked about the WWDC app being written in Swift they also explicitly mentioned that Swift will run on iOS 7 and 8 as well as OS X Mavericks and Yosemite.

[lcnmrn]

Swift code compiles and runs on 10.5 and up. Tested it myself.