The U.S. Air Force is developing a refurbishment program for its F-15 fighters, most of which are over 20 years old, so they can extend their useful life from 8,000 flight hours, to 12,000 hours. The U.S. Air Force wants to extend the life of some F-15s because of delays in getting the new F-35 into production. The F-16C, which was originally designed for a service life of 4,000 hours in the air, took advantage of advances in engineering, materials and maintenance techniques have already extended that to over 8,000 hours.
Aging combat aircraft are a worldwide problem. Earlier this year, the Chinese Air Force began retiring its first Su-27 fighters, or at least the airframes. These aircraft were bought in the early 1990s, and 24 of them have reached their service life of 5,000 flight hours. "Service life" is a limitation all aircraft have, although it can often be extended. In addition to the F-16 and F-15 extension programs, the U.S. A-10 also had its service life extended from 8,000 to 28,000 hours. Same deal with the B-52, which had its useful life more than doubled, via several refurbishments, to 28,000 hours. Engines, electronics and other components have different service lives. So the retired Chinese Su-27s were stripped of most components, for reuse as spare parts.
Russian warplanes have, historically, had short service lives. This includes all components, especially engines. The MiG-29 was designed to last only 2,500 hours in the air. A refurbishment program has since been developed to extend that to 4,000 hours. The MiG-29 was a watershed design for the Russians in the 1970s, who were beginning to build more sturdy aircraft on the Western model. Thus the Su-27s, which were designed a few years after the MiG-29, had the longer, for Russian aircraft, service life of 5,000 hours. Before that, most Russian aircraft were only good for 2-3,000 flight hours.
The Chinese Su-27s, which normally have two pilots assigned (a common practice worldwide) apparently allowed each pilot to get 120-130 hours a year in the air. That's less than Western pilots get, but twice what pilots used to get in communist countries. That's because these nations had Russian aircraft that would be quickly worn out if you allowed the pilots to fly them as much as their Western counterparts. But the Russians saw the error of their ways before the end of the Cold War, but not in time to re-equip their air force with pilots trained to a Western standard.