The phrase “too old to cut the mustard” was popularized by a hit song during the 1940s when military expressions were uppermost in the minds of returning servicemen.
Simply put, it means that one’s “salad days” are in the past.
Mustard is a mispronunciation of the military word muster, which means “inspection.”
If a soldier doesn’t “cut or pass muster,” he or she doesn’t make the grade.
In effect, the soldier fails to pass inspection, and that’s where the expression comes from.