Can I use APIs and Frameworks introduced in iOS 6 in an app that supports earlier iOS versions?

Is this only possible through workarounds and code such as:

[[UIDevice currentDevice] systemVersion]

Or would I be able to use updated frameworks, APIs, etc. in an app that supports earlier versions of iOS without specifying the version?

And also, would I be able to use Xcode 4.5, or would I have to stick to the current public release?

ios frameworks backwards-compatibility ios6
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