The B-52 first flew in 1952,they only built them for 10 years, the youngest now flying dates from 1962. But this is a one-off. A combination of a robust design that's useful for a niche purpose, and the insane cost of a clean sheet, replacement. Note that the Vickers Valient, a similar British strategic nuclear bomber that dated from the same era, only lasted in service until the mid-60's as they were basically falling to pieces. That could easily have been the B-52, had it's designers made some bad decisions.
It's interesting to compare this with the C-130 which first flew a little later, 1954, and is still being built. The time interval over which they have been building them is longer than the time interval between the Wright Brothers, and the first C-130 flight.
This gives rise to the interesting thought that in certain niche areas (dropping insanely huge numbers of bombs, landing 10 tons of cargo on a remote dirt airstrip) we have reached "peak aeroplane" and did so decades ago. Essentially, spending a huge wodge of money on a clean sheet design to do those jobs will never result in benefits that justify the cost. Far better just to tweak the designs we have with a few incremental improvements.
Civil aircraft don't seem to have reached peak as there are still improvements (in running cost) to be made, which justify new designs. "The average amount of energy consumed per mile, per passenger, fell by 74% on domestic flights in America between 1970 and 2010", according to The Economist. But presumably that will also eventually peak out in the future, eventually making brand-new civil designs pointless.