Good question...

How on God’s Green Earth Is the B-52 Still in Service? | WIRED

From the article:

Even with the modernization, the currently flying B-52s are all about 55 years old, about the age humans start getting calls from the AARP. This is where the over-engineering comes in. “The airframe itself remains structurally sound and has many useful flying years ahead of it,” the directorate official says. “Most of the B-52 airframes are original and their longevity is a testimony to the original design engineers.” In other words, they did a killer job making a durable airplane. 
Even the flight controls—the yokes in the cockpit, the seats, the control surfaces on the wings and tail assembly, the cable linkages between them—are largely the same as they were when they were built in 1960 and 1961. Of course, inspections are frequent, and the airplanes undergo heavy maintenance inspections every 4 years, during which mechanical and structural elements may be replaced as needed, along with possible replacements of any of each sample’s eight Pratt & Whitney jet engines. But for the most part, the crews in charge today have got their hands on the same BUFFs that crews touched decades ago. In some cases, recent crew members have been sons and grandsons of previous-generation B-52 crew members.

Yep, that's the biggest surprise:  that the airframes are still good.  I presume though that they don't pull G, and I would guess that is pretty much the reason you can't expect a fighter airframe to last anything like that age.  (AFAIK).