moish01, the infant mortality rate in those days (even until 100 years ago) was very high. Many children did not make it past 5. Those that did were obviously stronger. In addition, as some here have pointed out, if a person dies at 40, he won’t be dying from some cancers which takes forever to get a person.
ames, if your point were correct, there would be no point in davening or doctors or any hishtadlus. What is the point, if Hashem already decided. It is a lot more intellectually honest, as well as being more in tune with history to state that Chazal were really smart, nice, handsome, etc. people, but they were indeed people. That means that they did not know everything. That is one of the definitions of being a person. Furthermore, should a Jew not daven for a cure for cancer, because Hashem will just pick something else? He might, but that is not up to us, that is up to Him.