Models created in v16, can be opened in lower versions such as v15 or v13, (v9 or below are not possible) by exporting the file as a e2k file, via File>export, then open this file in a word processor such as notepad and change the ETABS program label and version number on the fourth line of the text file to the one you want to use.For example from v16 to v15 this will be:
PROGRAM "ETABS 2016" VERSION "16.X.X"
PROGRAM "ETABS 2015" VERSION "15.0.0"
Then you can save this file and open in the lower version via File >import>e2k file. Please notice all new features will not be compatible with lower versions