Our extensions work with Joomla 3.1.1 to 3.6.x, and with PHP 5.3 to 5.6, and PHP 7. Sites run noticably faster with PHP 7.
Support for Joomla 2.5 ended on December 31st, 2014 and we have been working towards ending support for our extensions in Joomla 2.5. We have already ended distribution of the Joomla 2.5 versions of our e-commerce extensions, and will remove most of the rest by the end of 2016. We'll make an exception for Eyesite and will keep that available for 2.5 until the end of 2017, but without active support.
We strongly recommend that you keep your site up to date with the latest version of Joomla. From time to time significant security issues emerge, and sites that are not updated are quickly attacked once details of the problem are published. In general you can assume that our extensions work on new minor releases of Joomla, but if your site is critical to your business you should have a parallel test site where you can test upgrades before implementing them in your live site.
We will always offer our best advice, but if you want us to research a complex or time-consiming problem, it must be on the latest version of Joomla with the latest version of our product.
There's a table of Joomla versions and support dates at http://docs.joomla.org/Joomla!_CMS_versions