There is a hard line drawn at 2007. Before the file extensions were .doc/.xls/.ppt as examples. Starting with 2007 the extensions add a character (usually x), so .docx/.xlsx/.pptx for instance.
https://en.wikipedia.org/wiki/List_of_Microsoft_Office_filename_extensions
If you have an older Office version you need a compatibility pack to open newer file types:
https://support.office.com/en-us/ar...-of-Word-8FE47805-64A9-4CC5-A115-B148625FE043 You also may not be able to access all the features possible with newer versions and as time wears on the functionality within newer file extensions can expect to degrade more under older, deprecated versions of Office.
Thus it becomes a question of how much of your Office is for you and you alone.
The more you import from other sources and export to other users, the more you need to be at least at Office '07 and above. If your use is limited to yourself you could run older versions of Office and be fine.
FWIW, I'm on Windows 10 and I'm running Office 2010 on my laptop. For my tablet I'm on RT 8.1 and have Office 2013 I'm not quite ready to trade up but I'm getting close. I almost pulled the trigger on a Home and Student install of 2016. I still may before the year is out. Isn't it nice to be current? It keeps your content relevant across devices and for a majority of users who may have to interact with that content at some point.