The answer to this question can be found on Meta SO: Someone with <2000 rep edited my answer
It can also be found in this blog post:
http://blog.stackoverflow.com/2011/02/suggested-edits-and-edit-review/
According to the blog article, anyone can edit posts older than 12 hours as a suggested edit that requires approval, but the edit option is hidden for posts newer than 12 hours, unless the user has the privilege for editing questions and answers. This is 2000 reputation on the released sites but may be lower on public beta sites and even lower on private beta sites.
