What is the life limit due to metal fatigue in large bombers?
Metal fatigue and fatigue corrosion are and have been a problem since the type's introduction. They always fatigue and corrode, they always undergo expensive repairs due to corrosion. DIEGME degradation of fuel tanks has been a particular culprit, and it has been reduced with new coatings. 
Bombers aren't pressurized, so the whole fuselage doesn't fatigue the way it does for airliners. The hard limit for wing spars fatigue can run into the centuries, considering that these planes don't fly all that much.
100 years is a bit of an exaggeration. The oldest B-52 in service come from the late 50s to 1962, when the production ended. So at retirement time the planes will be just 90 years old. There is a lot of B-52s sitting idle in storage, since they used to be considered strategic and fell under SALT cuts, and they're available both as spare part sources and as restorable airframes.
Why shouldn't a 100 year old plane fly? The USS Constitution is 220 years old and she's still sailing the seas, a considerably more hostile environment. It took a lot of restoration, but not quite to the extent of the Theseus' ship. Not everything is built with planned obsolescence like cars or iphones; ships and planes are built to last, as long as the owner spends the money needed to keep them up.
Keeping the old planes intact may not be cheap, but new bombers are far more expensive. The B-1B is half a billion a piece, so is the Tu-160. At the same time, most of the work the USAF does is milk runs in a threat-free environment. It doesn't call for advanced supersonic or stealth machines, it calls for something that's easy to service and cheap to run. The B-52 fits the bill better than any other bomber the USAF has. The only extant competitor is the Tu-95.
The B-52 can in theory be replaced by something even cheaper to run. Plenty of Boeing manufacturing capacity will sit idle for a long time, and, given a supply of olive drab, they'd fit the job even better. But this can't happen, because every military procurement program since the WW2 begins and ends with feature creep, doubly so in the Air Force. Only existing inventory can be kept as simple as it is.


All of this essentially leaves no choice but to keep up the current planes. Aluminum is cheap, it's silicon that's expensive, and B-52s have very little of that.