The B52 first was first rolled out for production use on 18 March 1954. The "...long-rifle of the air age..." -- Nathan Twining. It has not been manufactured since 1962. They are still in active military service, the ones that remain, and continue to see upgrades and evolving mission scope. 100 years of service is feasible.
Cost per flying hour of a modern B52: $70,000 [1]
1. https://www.airforcemag.com/article/re-engining-the-b-52/