The installer has this timestamp: 1373509356 (Thu, 11 Jul 2013 02:22:36 UTC). So the last version probably isn't too old. (The timestamp of course is not necessarily correct)

Wikipedia talks of its birthday here: http://en.wikipedia.org/wiki/WebM

Find the source code and you will have your answer. Click on the installer help link on the download page - I could not find it there.

younger than IE9.