One of the more severe punishments meted out by the Torah for certain misdeeds is the punishment of karet. A person who eats on Yom Kippur, for example, would be liable for that punishment. It is interesting that the Torah never chooses to define what exactly karet entails, and the baraita in our Gemara distinguishes between karet and mita be-yedei shamayim (a death penalty as carried out by the heavenly court) by saying that someone who dies at age 50 has received karet, while mita be-yedei shamayim is when someone dies at age 60. Rabba argues that karet actually is defined by death between the ages of 50 and 60, but the baraita does not emphasize that because it wants to honor the prophet Shmu’el who passed away when he was 52.
To emphasize this point, the Gemara relates that Rav Yosef threw a party for the Sages when he turned 60, celebrating the fact that he has succeeded in living beyond the stage in life that would have indicated the punishment of karet.
The Talmud Yerushalmi brings a proof-text for the idea that karet is indicated by death at the age of 50 from the passage (Bamidbar 4:18) where the Torah expresses concern for the lives of the levi’im, who work in close proximity with the mishkan and its utensils – a closeness that can potentially bring death to the person who comes into contact with those things inappropriately. In expressing this concern, the Torah uses the expression al takhritu – do not allow them to suffer karet. Given that the work of the levi’im in the mishkan ends at age 50, the implication is that anyone who dies before that age may have succumbed to the punishment of karet.
The Ran suggests that the underlying reason for this is that we assume that a normal life-span is 80 years (see, for example, Tehillim 90:10). For the first 20 years, heaven does not hold an individual liable for his actions. A person who does not live past 50 has not succeeded in living even half of the remaining 60 years, which indicates that he is in the category of those evil people described in 55:24.