We added a lot of stuff in Lollipop to make Activity Transitions work well. It is possible to do Activity Transitions manually (in earlier versions), but you will encounter problems in some cases and the return transition is likely going to work only in the simplest case. Chet Haase has a devbyte on this here.
Animations between fragments were possible earlier, though the API isn't as easy to use as in Lollipop's Fragment Transitions.
We thought a lot about back porting transitions to earlier versions in the support library. It may still happen. If so, we should also be able to do something for Fragment Transitions.
