GuyMac:
I don't know if you can achieve what you want with a rule, but you can autoarchive. You can set it not to prompt and to delete old emails instead of keeping them.
First you turn on autoarchive in Tools|Options|Other tab.
Set the frequency, the prompt user option and the option to delete expired items or not.
Then you have to set the properties of individual folders. Right-click the folder on the shortcut menu (left-side) and choose properties. Go to the autoarchive tab, set the age for items to expire and choose to delete them or to archive them to a file.
Hope this helps!
Desiree