It seems like there used to be 5 minutes during which a post could be edited before it was indicated that it was edited with an icon, but that time has apparently been decreased?
Not that I know of. This setting is still 300 seconds
For (n) seconds after posting, editing will not create a new version in the post history.
There is also another criteria that the forum s/w uses and that is how much of the post was changed. And is set to 100 characters for trust level 0 & 1 and 400 characters for trust level 2
Maximum number of character changes allowed in editing grace period, if more changed store another post revision