Traditionally, Autodesk has updated the file format in AutoCAD every 3 years. These milestones meant that there was a major change to the format in 2007,2010, 2013 and so forth. In 2014 for instance, you were able to pick 2013 or earlier and in 2015 you would have the same options.
This release though, Autodesk has decided to not advance the format for 2017. You’ll note in AutoCAD 2017 that there is no option to save-as a 2017 version as you might expect.