It is obvious they are using South American crews, as the B707 are almost inexistent and the DC-8's even in South America are vanishing.
So older pilots do not want to upgrade to newer aircraft because of the high cost and prefer to find place where they can continue flying an old bird.
In Europe you won't find any pilots anymore able to fly these birds (they all retired yet) and even in Africa it becomes a huge problem as most cargo-pilots who get a day older try to fly local pax flights.
It seems ACE is looking for other airframes to extend their fleet.