I have never read any official rule about necroing old posts and considering the forum is only a bit more than 2 years old, the old posts aren't really that old. Other places I occasionally haunt seem to put 3-4 years as the semi- or fully official "this post is closed to new replies due to inactivity" text.
I suppose if the responses were to issues that no longer are relevant that could cause confusion, but the game hasn't change too much since v1.4 (eh...... or not at all if you're on mobile....) to make a lot of topics obsolete.