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?
Here are the system requirements for the past several versions,which answers your OS question:
So anything before CS3 is considered legacy by Adobe?
CS3 was version 8.0, which is five versions back. We have typically tested five versions back for project import. With the version coming out, we extended that to six.