To my understanding, there is a time limit. You can edit for a while, but then after a certain period of time, it 'archives'. The idea is that you don't want to go around editing old posts, because you could 'correct' something you had that others had commented on, and it might cause problems later. (Say, you claim one thing, are proven false, then edit your original post so you can later claim you were never wrong.)

edit: (See, you can edit ) I don't know about this board, but some also make it so that you can only edit your post while it is the 'newest' post in the thread. As soon as someone else posts, you lose the ability to edit. I don't know for sure which method is used here.