Sage have updated their supported versions list and are displaying the versions supported as well as retirement dates for the past 4 releases.
As shown in the image above the following versions of Sage 100 ERP are supported as of December 2014:
Sage 100 ERP 4.5 (last date for product updates – phone support ends 9/30/15)
Sage 100 ERP 2013
Sage 100 ERP 2014
Sage 100 ERP 2015 (projected release date – April 2015)
You should note that the last date for product updates and hot fixes may be a truer measure of when you should ensure your Sage 100 system has been upgraded. If there is an issue requiring a program fix and the version that you use is supported by phone but not product update or bug fix then you may be forced to quickly update to a later version if a program fix is deemed necessary by Sage.