For better or for worse, Xcode versions are in rough lockstep with Mac OS X versions. Old versions of Xcode won’t work on new Mac OS X releases, and new versions of Xcode won’t work on old Mac OS X releases. This is because Xcode takes advantage of new Mac OS X features, and Apple is relentless about dropping obsolete features from Mac OS X, thus breaking old versions of Xcode.

Xcode 3.2 - min Mac OS X 10.6, still supported PPC

Xcode 4.0 - min Mac OS X 10.6, min OSX SDK 10.6, min iOS SDK 4.3, drops PPC

Xcode 4.2 - min Mac OS X 10.6, drops GCC support

Xcode 4.3 - min Mac OS X 10.7

Xcode 4.6.3 - min Mac OS X 10.7.5

Xcode 5.0 - min Mac OS X 10.8 (?)

Xcode 6.0 - min Mac OS X 10.9.4

Xcode 6.2 - min Mac OS X 10.9.5

Xcode 6.3 - min Mac OS X 10.10.0 (?)

Xcode 7.0 - min Mac OS X 10.10.4

Xcode 7.1 - min Mac OS X 10.10.5

Xcode 7.3 - min Mac OS X 10.11

Reference

xcode-clang-vers gist

Xcode (Wikipedia)

Xcode downloads