We do not have the system set to delete old posts, as over the years we have seen some users start flame wars, make brash claims that get shot down by others, and then they want to remove their post that started it all -- leaving the boards looking crazy as readers then look at things completely out of context.
We have our system set to give users a way to preview their posts. If a post is made, then for 30 minutes or until someone responds to it, there is an "Edit your post" link available when you return to your post. But there is no provision made for users to delete their posts.
We have a policy statement of our terms and conditions and users are free to read them before joining the site.
I hope that I have covered your question clearly.
Remember: Burt Bacharach lied. What the world really needs now is an undo button.