Moderator's Note:
Please Note: there is nothing wrong with reviving an old thread intentionally. In fact, we encourage people to thoughtfully revive threads, when the new information is both relevant and appropriate for the thread.
The warning is there so people realize that the thread has been inactive for a considerable length of time when they make their post. For some threads - such as threads involving market conditions - the amount of time that has gone by makes a current response of little use. But in many other cases, the inactivity matters little.