What is meant by "the most decrepit stage of life" in the noble verse: "And among you is he who is returned to the most decrepit stage of life," and does it refer to senility or the end of life, and what is the reason for people's variation in that?

"Ardhal al-'umur" (the most decrepit stage of life) refers to reaching old age and senility, where a person becomes physically weak, mentally frail, and poor in understanding, returning to a state similar to childhood, thus becoming a burden on their family. This senility may afflict a person at a certain age, but some people may be spared from it even at a very advanced age.