Plugin Age

You can view this on each plugin’s page. For example on one of mine at http://wordpress.org/plugins/alligator-popup/ there is a block on the right above the ratings that features the following text:

Requires: 3.6.1 or higher
Compatible up to: 3.8
Last Updated: 2013-12-24
Downloads: 2,996

You can also see the last update when you view the plugins in a list eg: http://wordpress.org/plugins/browse/new/

or perform a search:
http://wordpress.org/plugins/search.php?q=audio