JHMO... Old threads are helpful as a reference. It's just that rules, policies and strategies can change over time. Very old threads can be confusing/misleading to anyone who isn't paying close attention. Once an old thread is resurrected, I've seen folks start replying to the original poster or quoting old posts made in the thread.
But I understand not wanting to prevent newbies from posting on the forums. Too bad posting on an mothball thread couldn't trigger a new thread with a link to the old.
I agree--wish there was a way to indicate it's a 'mature' thread. In the past, moderators have altered the subject heading. Just one recent example of retrieving old threads that many of us like is Cat's Thanksgiving Turkey Brining Recipe.