I think that is intentional; edits for mere mortals are there to quickly fix mistakes you notice after posting. Changing forum history, even though every regular user edit would still be marked, arbitrarily far back in time may be considered too powerful and therefore only reserved for mods. Just guessing, though.
Thanks; I'm not sure but I seem to remember that it was possible before to edit posts that were older than 2 years and that I already did that occasionally.