The U.S. Air Force mission capable or “readiness rate” (percentage of available aircraft able to do their job) varies by type and technology.. Age has less to do with it than you might think. For example, the 2015 rates for fighters are (compared to 2010);
F-15C, 71 percent same as it was in 2010
F-15E, 71 percent down from 72.5 percent in 2010
F-16, 74 percent down from 75.4 percent
F-22, 67 percent up from 60.9 percent
A-10, 77 percent up from 70.5 percent
Note that the most recent aircraft, the F-22 (introduced in 2005) had the lowest readiness rate. That is not unusual. The F-22 rate finally overcame the problems of being a new aircraft with problems are still being uncovered. F-22 still has another problem in that the many stealth features of this aircraft require attention and provide more items that can break. Meanwhile the older fighters, most built before the Cold War ended in 1991, have aged well. The A-10 dates back to the 1970s but after several overhauls and upgrades continues to have a high readiness rate.
For bombers, the rates are;
B-1B, 47 percent up from 43.8 percent in 2010
B-52H, 72 percent down from 74.6 percent
The B-1B is cursed with additional components (especially hydraulics) that enabled it to fly fast and low. That capability is not used anymore, but the equipment is still there, and when any of it breaks, the aircraft doesn't fly. Meanwhile, the ancient, but relatively simple, B-52 has the highest readiness rate, and is the cheapest to operate. Nevertheless the B-52s are being worn down, not by years but by heavy use since 2001.
Transports also are relatively simple in terms of tech, and their readiness rates show this;
C-130J, 80 percent down from 82.3 percent
C-17A, 85 percent up from 84.4 percent
CV-22, 56 percent up from 54.3 percent
The CV-22, the first tilt-rotor transport is paying the price for being first. Its tech is also complex, and that takes a lot longer to tweak into a high state of reliability.
Aerial tankers are basically transports, and quite elderly;
KC-10A, 83 percent up from 74.8 percent
KC-135R, 75 percent down from 81.1 percent
KC-135T, 80.4 percent
The readiness rate for these old, and heavily used, aircraft does not reflect the large number (up to 20 percent) of aircraft that are pulled for major rebuilds. Thus the most decrepit tankers are not counted, keeping the readiness of aircraft in squadrons at a high rate. But these increasingly frequent rebuilds are not only expensive and reduce aircraft available but are often unpredictable. As aircraft get older their specific failures become less predictable.
UAVs are simple aircraft, and their high readiness rate reflects this;
MQ-9, 92 percent unchanged from 91.9 percent
MQ-1, 92 percent down from 93 percent
Out of some 5,000 aircraft and helicopters in the U.S. Air Force there is one model, the C-21C transport that had a 100 percent readiness rate. The C-21Cs are actually the eight ton Lear Jet 45A aircraft used mainly as executive transports (“business jets”). The air force has 31 of them and they can be configured to carry eight passengers or 1.4 tons of cargo. But the C-21Cs are now only used for moving personnel, mainly senior officers, around which may explain their high readiness rate.
American warplanes have the highest readiness rates on the planet. That isn’t cheap and requires decades of effort to develop the people, equipment and techniques to make it happen. One form of American military aid much in demand (but not sexy enough to report on) is to have American experts teach foreign troops how best to maintain warplanes.