For what it's worth, Patroni basically got it right in his post. To expand a bit, I would add that the poster who taked about mixed missions also rounded out the picture. The only other issue that hasn't been discussed is dispatch reliability.
Old planes: lower dispatch reliability (usually, although not always); lower capital cost; higher maint cost; higher fuel cost. This means that the thing has a high operating cost per hour, but the low ownership cost offsets the high cost per hour if it isn't flying a lot. This means: you use this plane for missions where it's going to sit a lot, for example, out and back to a hub once a day. You can also have a few spares. You also use it on missions where, if it breaks, you can flag-stop another plane or run a plane down there during the off-hours so you don't miss its one turn per day. So, for example, it can be downright economical for that once-a-day ORD
New (and new-er) planes: higher dispatch reliability, much higher capital cost; lower maint cost; lower fuel cost; sometimes lower crew cost. This means that the thing has a lower operating cost per hour, but the high ownership cost offsets the low operating cost unless it is flying a lot. This means: you use this plane for missions where it's going to be flying its tail off, for example on long transcontinental runs (where you do, say two trips to the hub per day on 4.5-hour legs) or intercontinental runs, and where you're in more difficulty operationally if it breaks. SDF
, for example.
An optimal fleet will have a mix matched to the missions. Where some component of business is seasonal, you want a low-capital cost component of the fleet available to permit the ability to handle seasonal peaks. You can also (as UPS, FedEx, etc. do) bring in charter lift to help with the real seasonal peaks.
You also have a middle ground between "old" and "new". For example, ABX
Air flies 762s on DHL's transcontinental routes. These aircraft have a lower capital cost than new airframes, but still have good dispatch reliability, good fuel economy (compared, say, to the DC8 and certainly to the 727), a 2-pilot cockpit, modern avionics, a reliable source of rotable parts, and lots of places around the country and world that know how to fix them. The MD11 is capable of a great deal of lift and range on 3 engines, and has proven to be a very successful freighter because its lift and legs come with a size that's right for a lot of UPS and FedEx intercontinental missions, and its rather-quick demise as a passenger carrier gave it a capital cost that was optimal for conversion into a freighter; no need to haul around the volume and weight of a 747 if you don't need to do so.
Carriers like Evergreen, Kalitta, etc. have some highly-seasonal contracts, as well as contracts which are much shorter in years than the typical financing for a newer aircraft. Accordingly, lest they be stuck with a bunch of new airframes that they don't have long-term contracts or short-terms uses for (in an economic downturn, for example) , they tend to use older, lower-capital cost frames (so they don't end up like Atlas Air several years ago, for example, where they had a bunch of shiny new 747-400s coming off the line and nobody willing to pay them to fly them). This does, of course, sometimes limit their dispatch reliability, and thus the kind of business that they can normally get. One way to handle having a newer fleet if you don't have contract lengths that match your financing lengths is to stagger lease expirations, as World Airways did with its MD11s. That way, you have some flexibility to downsize if you need to without penalty (because you have to assume that if you can't sell the service, there isn't likely to be someone else wanting to snatch up your lease at the price that you're paying for it).
Some posters have also made observations that echo the sentiment common in the freight business: "Boxes don't bitch." And while it's true that a box doesn't care about the age of the aircraft, that's a truly secondary consideration. If the aircraft is clean and nicely-painted where it's sitting where the general public can see it, it sends the right message from a corporate-image point of view; almost nobody will know its age or condition. Some of the crappiest bucket-of-bolts freighters can look mighty nice on the exterior.
Hope this helps.