@rookwood We only keep the current and previous two versions of SketchUp available on our site for download however you may be able to download an older installer on a download site such as http://download.cnet.com/
As @DaveR mentioned we’ve made numerous changes in newer versions which aren’t available in SketchUp 2013. Unfortunately there is functionality from SketchUp 2013 and older which references Google specific functionality which we no longer have access to so we couldn’t continue using those features moving forward. We strongly encourage all customers to move to a more modern version of SketchUp for the best performance.