As Michael already cleared, projects are not backwards compatible. If the folder structure in the Mediapool is what you wan to preserve, you can use the Media Management function.
Oper up the said project in DVR18, go to File and select Media Management. Set that up as shown in the screenshot. Create a leading directory for the location. This will create a copy of all your project media, preserving the folder structure you created.
Then, in DVR16 open a new project, and in the media page locate the directory you copied the project into. There, simply select all the folders and right-click on them, then choose “Add to Mediapool (create bins)” and that should be it.
Then, as Michael suggested, you can use an XML to bring in your timeline and CDLs to transfer the grade.
Hope this helps!