Ok, here's the what's up on necroposting. The staff frown upon necroposting when a thread is several months old, say at least 2-3 months old. Depending on the post made that revived the thread, they may or may not close the thread. The older the thread, the higher the chance they will close the thread, regardless if your post isn't spamming it.
And I still dont know why they close the thread just because its old.