Well, not quite. The time of making is irrelevant; it's the time of last activity that determines whether you are necro-posting or not. If the thread was made in 2008 but has had steady activity from then until now, posting in it is not necro-posting. If the thread was made in 2008 and its last activity was in 2010, then posting in it is necro-posting.
I think the minimum time for subsequent posts to be considered necromancy is two-months.
However, if the necropost seems important and the poster explains why he or she made the post, people generally will not complain. For example: a mapmaker bringing to life his old unfinished project and releasing it, or something like that.
I hope I helped!