If you want to ask people not to submit old content, I'd suggest going beyond the list and coming up with a compelling reason why old content being reposted and upvoted and discussed vigorously is detrimental to the HN community as a whole and not simply annoying to a few old hands.
Some of that discussion is a complete retread, of course, but some of it is brand-spanking new and reflects the fact that both the people and the times have changed, so the ideas will also change.