Another option you have is to use Hazel, which is built specifically to monitor files and folders and then act on them based on criteria you set up. Many KM users also employ Hazel, and the two apps work well together.
I have several Hazel actions that move and/or rename files after a period of time. For instance, I take a lot of screenshots, so I have Hazel actions that keep screenshots I've taken in the last 48 hours on my desktop, but anything older gets moved to an archive and sorted into dated folders (year and month). Other types of files that land on my Desktop, Downloads, and dozens of other folders have similar do-something-if-file-is-older-than actions.