How could a 45 years old DC10 be still in service with one of the largest cargo airline?
The DC-10’s have all been converted from DC-10’s to MD-10’s. The last conversion was in 2007, I believe. The entire aircraft wiring was replaced, the avionics package converted to glass cockpit and other structural work was also performed. As a result, the airframe was “zero houred” upon completion of the conversion.

Although they’re getting older, they’re still warhorses and their reliability is still good. 

Also, cargo operators don’t put nearly as many cycles on their aircraft as passenger carriers. As a result, there is a lot less wear and tear on the aircraft.