Rule to not to allow content editing after a fixed amout of time

I am trying to create a rule (in 6.x) for a CCK node type edit form. I am trying to create a rule for Node edit form is being built.
If the content previously created is X days old I would like to deny user editing the content. But if it is less than X days old then the editing should resume as normal.

Any Ideas/thoughts? Am I on the right track? or should use a different approach. I prefer not to trigger the rule for each page view (and disable the edit node tab altogether) as that will be too costly in processor type. Rather want to trigger a message when there is intention to edit.