Most of our extensions now require a minimum of Joomla 3.7.0 due to our continuing preparations for Joomla 4. If you use an older version of Joomla 3 we can provide old versions of our paid extensions on request, but we no longer supply any extensions at all for Joomla 2.5 or 1.5.
Most of our extensions now work with Joomla 4 Alpha and all of them will be ready for Joomla 4 well before its release. According to the Joomla Project Roadmap, a stable release is scheduled for the 4th quarter of 2018.
We strongly recommend that you keep your site up to date with the latest version of Joomla. From time to time security issues emerge, and sites that are not updated may be attacked once details of the problem are published. In general you can assume that our extensions work on new releases of Joomla, but if your site is critical to your business you should have a test site where you can test upgrades before implementing them in your live site.
There's a table of Joomla versions and support dates at http://docs.joomla.org/Joomla!_CMS_versions
We develop and test our extensions on PHP 7, although we also maintain compatibility with PHP 5.3 to 5.6. If you have not yet upgraded to PHP 7 you should do so. Sites run noticeably faster with PHP 7, and Joomla 4 requires PHP 7 as a minimum.