Comments can be edited until the edit window runs out (2 hours). It's necessary for discussions to congeal after a while, because it's confusing to readers and unfair to repliers when an earlier comment removes what a later comment replied to. Allowing edits for a while, and then not, is the tradeoff here.

If users want a comment edit after the window has expired, they can always email us at hn@ycombinator.com.