look in the forum settings, there is a setting for "Allow Users To Edit Their Posts For Up To This Many Minutes", it defaults to 60 minutes. If you really want people to be able to edit their posts forever you can change it to -1. Myself I don't like it when people edit older posts or change the question after it has been answered, I think one hour is more than enough time to allow. There is also a setting to close threads after a certain number of days so that no new posts can be made on the thread, by default it is 365 days or 1 year, again if you want no limit you can set it to -1.
Of course users with edit rights on the page or forum instance (ie moderators) can edit any post at any time.
Hope that helps,