In addition to the great post from Error 601, keep in mind that a lot of freighters have low utilization, i.e. they sit around on ramps a lot of the time (which is part of why they can fly for so many more years). Less time in the air means less fuel burned than passenger aircraft. Obviously the fuel burn per hour of flight is still going to be higher on the old planes, but the up-front cost to acquire them is also a lot lower than the cost to acquire newer, more fuel-efficient planes. For freight operators, that math typically works out in favour of the older planes.