Wow, what amazing research...
You almost wouldn't realise this has been known about for years.
Startpage search for the following RBLs, Day Old Bread, SEM have several age lists 5,10, 15 day etc, SURBL Fresh. If your firewall or mail server can use RBLs you can filter these or score them in Spamassassin.
Biting the hand that feeds IT
