the way I understand it, dark cutters occurs because of a large release of adrenaline (or it's counterpart in animals). 

As to the aging of grain-fed beef, my dad has always instructed the locker to hang it until the morning the butcher can sniff the carcass and detect the beginning of mold with his nose, then cut it up, usually somewhere around  day 15-19.  I think a carcass could hang longer if it was the only carcass in the cooler, because as fresh carcasses are brought in to a big cooler, warmth and humidity changes are also introduced.