There's a time-limit set in the forum software that once it times out posts are no longer editable. I'm not 100% sure what the limit is, but I believe that it's 10 minutes. One of the admins will be able to confirm.
After the time limit has expired, only mods or admins can edit posts.
If it's just a minor thing, like a typo, not usually something to worry about. If you mis-stated some information, you can just make a new post in the same thread to clarify. If it's really serious, you can PM someone on the staff to help you out.
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