Microsoft Dynamics GP 2013 came out of mainstream support and into extended support as of 1oth April 2018; end of mainstream support means you will no longer get statutory updates such as the addition of the Invoice Receipt Date in the January 2017 hotfix which applied to the UK or year-end tax updates for the US and Canada.
I always recommend to client to look to upgrade every two to three years; the last few years have seen many improvements and additional new functionality added to Microsoft Dynamics GP (2013, 2013 R2, 2015, 2015 R2, 2016, 2016 R2 and 2018) as well as bug fixes.
By not upgrading regularly, you not only miss out on all of these fixes and new functionality, but you also make your eventual upgrade longer and more complex due to the number of versions which need to be upgraded through.
You can review the product lifecycle on the Microsoft Lifecycle Policy – Microsoft Dynamics GP page.