In general, the ability to edit a post runs out after time. This is largely to prevent discussions from being edited away so that they no longer make sense. (It's rather confusing to read a reply to a post that has been edited into something utterly different.)
Talking about whether or not we can edit something is considered "discussing moderation." If we discus moderation in the main forums, it tends to derail a
thread to be a discussion about moderation, rather than about how to grow plants (or build a
rocket stove, or mend a shirt, or whatever).
So, we like all moderation discussions to take place here in Tinkering with this Site, that way, the main threads aren't derailed.
One of the main goals of this site is for it to be a source of useful,
perennial information. That's why we don't discuss moderation outside of this Tinkering forum, and why the edit button automatically disappears after some time.