I don't think a rule would work for this task. Try running a search for the older messages using date-ranges, then delete the results. If there are a lot of them, it may take some effort to get rid of them all. I just tried searching for an entire year, got 293 hits, and am able to "select all" and then drag and drop them to the Trash folder.
How about a rule to delete all “unread”? I can enable it once. And then periodically.
I don't think so. If you go into the Filters in Settings, in Conditions and Actions there is nothing like what you are suggesting.