Teamviewer 6 licencing

Hi all, i used teamviewer a few years ago at my old job and i'm sure we paid annually for the licence.  At my new company we don't really use teamviewer but are looking into it.  My boss says he has a teamviewer 6 licence from 2011, would this still be valid?  I always thought it was an annual cost?

Apart from all that i dont even know if version 6 would be compatible with Windows 10.