Very true. For several years my mother developed (nearly instantaneously!) pulmonary edema if she went even 20 feet above 12,000 feet. Very strange that the altitude at which is always happened was so well defined, and also very strange that the condition developed in a matter of minutes rather than hours.
But after a few years when she was in better shape she had no problem climbing the Maroon Bells in Colo (both 14k). So personal differences can be huge, and personal tolerances can change over time.