I couldn't find in Apple's documentation (Cocoa Auto Layout Guide) what is the lowest iOS version that supports this feature, nor any posts related to iOS... maybe it only depends on Xcode version, and not on iOS version? I need to develop an iOS app that should be compatible from the latest iOS version down to 4.3
Thanks
Autolayout was introduced at WWDC 2012 and is supported starting on iOS 6. It'll definitely not work with iOS 4.3.
I strongly recommend you to think about if you really need to support iOS 4.3 these days. We're probably very close to have iOS 7 now and you'll be years behind (technologically speaking) supporting iOS 4.3. I understand apps that are currently supporting iOS 4.3, but I don't think it's a good idea do start developing an app now with iOS 4.3 support in mind.