Sorry, I didn't really know you weren't supposed to do that. Now I know not to do it again. Thx
It's not that you can't post in old topics. It's just that if you do, you had better have something to actually add to the conversation. If a thread is more than a week old (see the date of the last post in it) and you really want to post something important in it, then read the whole thread to make sure it hasn't been said already. If it has not, then you should say something like "I am bumping this to say..." so that people know that you have intentionally resurrected a dead conversation to re-spark interest in the topic and direct focus towards your comment.
But generally, if you have something useful to say, it's safer and usually just as acceptable to simply start a new thread.