I know this topic comes up every so often. I just ran into it today where I was trying to open an AE 5.0 project in CS6 and it just won't.
EDIT: I installed CS5 and it still won't open. Geez. Installing CS4 now...
Is there a handy dandy chart that shows how far back each version of After Effects can open AEP project?
And what's the earliest version of After Effects can I install and run on a current Mac with Mountain Lion (no Rosetta)? What about Windows 7? Win8?