If you have a question *that has not been answered* in an older thread, why not open a new one? I think people usually only complain about not searching for old threads when a common question has been asked that has already been answered before (counter example?).
Hence, I'd say if you have an unanswered question and the only related threads are more than a few weeks old, open a new thread. If somehow an older thread is still so relevant that it deserves mentioning, just link it in your post. That should make everyone happy (as fas as possible).
The problem with reviving old threads is that parts of the discussion might simply be not relevant any more, and it's simply annoying to read several posts wondering what the hell people are talking about only to realize that you just wasted your time with reading obsolete discussions. Moreover, sometimes people don't realize the age of the thread at all and start to reply to things that don't make any sense any more or to people who might not even be in the forum any more (see
this thread for a recent example).
Hence, I'd say even if an old thread is perfectly relevant, open a new one (and link the old one) because it's just confusing otherwise. If there was a forum software that would make it more obvious how old posts are (e.g. via a colour scheme) on the other hand...