Most communities allow you to edit for a while after posting and then stop. The length of that window is set by whoever runs the site.
The reason it closes at all is that a conversation people have already replied to stops making sense if the thing they replied to changes underneath them.
If you need a change after the window has closed — a wrong link, something you would rather not have said — ask a moderator. Moderators are never limited by the clock, because they are not fixing their own mistake.