With all the tiny updates on Creative Cloud, now we are forced to update every photoshop in our office in order to share PSD files. This is ridiculous. There should be some basic backwards compatibility built in to the PSD files. With several people in the office, hardly anyone can be expected to be on the exact same version everyday. Bad job.
I am able to open a PS CC 2014 PSD in PS CS5--I created a new document and added a few brush strokes then saved it, so I think it depends on exactly what operations you’ve done to the document and stored in the PSD. Obviously some smart filters that are new in PS-CC aren’t going to be understood in older versions of PS.
There is backward compatibility built in - your PSD files will open back to Photoshop 3.0, or whichever version added support for the major parameters (size, bit depth, color mode) that you are using.
Newer features may be rasterized, or may present a warning that they cannot be edited - but the files will open and usually preserve their appearance.
Photoshop works very hard to make sure that our files are backward compatible.