Can projects compiled in older versions of Xcode run properly (or at all) on newer version of iOS?
For example, can a project compiled in Xcode 4.2 run on platforms with iOS 8.
Is this possible?
If You're not using removed API it should run OK. However some methods behave differently. You can have problems with UI (for example Alerts and Actionsheets now uses different API). If You targeted iOS < 7 then UI will definitely be messed up.
You can check API differences on Apple Developer site (it's iOS 7.1 - 8.0 diff, check the earlier diffs to be sure).
To sum up: if You've been following Apple recomendations while writting code in earlier Xcode for earlier iOS there should not be many problems. But You'll never know until You run and test it.
I'm definitely no expert, but from what I've seen, occasionally certain parts of code need updating/re-writing to allow Xcode to compile with a newer SDK.