Why, Moses’s own brother Aaron lived till 123, and our forefathers down to his time lived well upwards of 120.In modern times, Jeanne Calment beat the limit by over 2 years."If something extremely bad occurs, people forget all their usual daily worries and become totally preoccupied with this single, truly serious problem.
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(In fact, Noah was commanded to begin construction of the ark a full 120 years in advance (and on top of a mountain) so that people would notice and inquire – and perhaps Noah’s response would stir them to repent in time.) A second possible source is Moses’s lifespan.
Moses lived till exactly 120 (Talmud Sotah 13b) – and on top of it the Torah attests that his energy and vitality did not diminish in the slightest before that time (Deut. We thus bless people today that they be granted the same long, productive life of our great teacher Moses.
See also Ha’amek Davar who observes dryly that by then a person’s human faculties have virtually left him.
I also seen quoted from the Jerusalem Talmud that a person’s body will decay by 120, although his body may continue to function a bit longer than that (especially with modern medicine).