If the topic hasn't been posted in for a long time the box you type in will have a red colored message above the type box it saying:
"Warning: this topic has not been posted in for at least 120 days.
Unless you're sure you want to reply, please consider starting a new topic."
It gives this message because unless you can get a real conversation going, there's no point posting in something 120 days old or possibly years old.
This rule exists because people started to spam old topics just to increase their amount of posts.