You can only edit posts for up to 15 minutes after it was created. Paul explained why a while back, but alas, I can't remember the reason. EDIT: I still stick by this is the first forum I've ever seen have a limit, XD I've edited 2 year long posts on some forums
probely a security feature so if a user makes a innapriote post they cant change it sorry for louzy answer and spelling im on my psp.