No official rule but typically frowned on. There is definitely no rule against repeat topics (sufficiently separated in time) so if there's something new to add, a new thread possibly with a link to the old one if there were comments in it relevant to the topic at hand would be a good idea.
Really old threads probably have a lot of participants no longer present to respond, and may confuse infrequent visitors who don't check the dates into believing it's actually a recent thread.
Of course, it's not a hard rule. If you're pretty sure most persons involved in the thread are still present, and the thread isn't sorely dated, go ahead and dig it up. But I think if you have the judgement to know when that's a good idea, you won't need me to tell you.
"The devil I'll bring you," answered Hagen. "I have enough to carry with my shield and breastplate; my helm is bright, the sword is in my hand, therefore I bring you naught."