I wonder if you could create a dimension that tracks something like "days since added"....
So let's say that content was posted on Aug 1
On Aug 1, the value would be 0
On Aug 2, the value would be 1
On Aug 3, the value would be 2
...
On Aug 21, the value would be 20
etc.
Then you could use a classification to group items by this "days since added" value, as in, items between 1 and 3 are "new", and 8 - X is "recent", X+1 - Y" is "aging", "Y+1 and over" is "old"
Using your interaction metric with this recency dimension would allow you to see interactions by "new" or "old" content as a whole.
If you notice a growing trend of "old content" getting more and more accessed, you can break it down by the actual content identifiers to see if specific items are driving the traffic, or if it's distributed.