There are actually TWO folders where you can install AE plugins. The common practice to install plugins in app>plugins is unfortunately not the ideal location. Installing plugins into this folder will only allow the specific version of the software access to the installed plugins.
These days, it is not uncommon to have more than one version of an app installed on the same machine. This is to allow for backward compatibility while saving project files and to allow for a step back to a previous version; just in case the newer version is unstable or on its own or in conjunction with 3rd party plugins.
A better location for installing AE plugins is in the "Common" folder that is found in a folder, one level above the application folder. Within this folder named, Common, you will see sub-folders with numbers, used as names. 7.0 is the folder designated for versions of AE within the CC brand. Technically, what this means is that as long as you install plugins into this folder, they will be accessible to all versions of AE CC.
I should make it clear that the folder structure within the OS, I have alluded to are relevant to Windows. The Mac structure/location of this Common folder may be different. But the numbered sub-folders within the Common folder should provide you with an indication as to whether you are looking at the correct folder.