Additional to this, I think even when someone is genuinely engaging with an old post, there are times where it is and is not "appropriate" to raise a thread. Not severe enough that it should be a rule, but an idealistic "best forum-user's practice" sort of thing.

If the new poster wants to engage in further discussion on the same subject, that's a good reason to bring up a discussion again, especially if the situation hasn't changed and the subject is still relevant.

If it's just to say "that's funny" or "I agree" without adding anything new to the discussion, it's probably not worth waking the thread up just for that.

And if people are talking about the lack of an obvious feature you know is in the game, it's really important to check the date before you helpfully inform them that it exists.