Joomla doesn't automatically get rid of old file in the cache, though you can clean the cache from administration. This simple script can be run via a CRON job to purge old files periodically from Joomla's cache directory. While it shouldn't pose a security risk, it is recommened that this script be run outside of your public directory, or at least in a password protected directory. You must edit purgecache.php so that it contains the correct path to Joomla's configuration file.