20 years is young for a cargo fleet. Passenger airlines in developed countries often aim to have an average fleet age younger than that, but the planes are built to fly for 20, 30, 40 years and sometimes more.

And remember that the maintenance procedures, mandated by law, for commercial aircraft are kind of insane. The analogy to a car (since everything has to have an analogy to a car) would be:

Every time you're about to get in the car and drive somewhere, first you do a full walk-around inspection of the exterior, then inspect all the lights, vents, wipers, horn, and so on. If anything is in less than perfect condition, you don't drive the car until it's been fixed.

Every ten days, you drop the car off overnight at a garage. They do a full fluid check, tire check and rotation, alignment, lubricate everything, and if they find anything less than perfect it gets fixed before driving again.

Once a month, oil change, filters replaced, new battery. If there's visible wear on the tires, replace them.

Every two years or so, you pull out the engine and replace it.

Every five years, you have the car completely disassembled and reassembled, replacing any part that's even the slightest bit worn or fatigued or shows any detectable amount of corrosion.

If you maintained a car this way, the car would last damn near forever. This is how planes are maintained, and is why they last as long as they do.

20 years isn't that much for an airframe, airframes are usually calculated by cycles (landing/takeoff) and flight hours.

The newest USAF airframe for it's KC135 tanker which is based on the 707 rolled out in 1956 which makes it almost 60 years old.

The average age for the 767-200 in AA is 25 https://www.aa.com/i18n/aboutUs/corporateResponsibility/prof...

So 20 years for a cargo aircraft especially if was converted from a passenger jet and overhauled isn't really that much.

Fedex's average fleet age is 22 years. Lots of cargo operations are run on used aircraft converted for cargo use.