It's not so much how old, but whether they've been well maintained, used in exercises for training, and updated with more modern weapons and equipment.
Our B-52 fleet started out in 1952 (72 yrs ago).
"The B-52 completed 60 years of continuous service with its original operator in 2015.
After being upgraded between 2013 and 2015, the last airplanes are expected to serve
into the 2050s.
These were once the third leg of our nuclear triad -- Subs, Missile Silos and Airdropped nukes.
Most B-52 pilots and maintenance personnel are retired or dead. The people flying and fixing them today were probably born 50 years after the aircraft first flew, give or take.
"There are 72 aircraft in inventory as of 2022;[9] 58 operated by active forces (2nd Bomb Wing and 5th Bomb Wing), 18 by reserve forces (307th Bomb Wing), and about 12 in long-term storage at the Davis-Monthan AFB Boneyard."