Musar zu Bamidbar 4:51

And [part of] the content of excision is the shortening of years; like the matter that is stated (Proverbs 10:27), “but the years of the wicked will be shortened.” But there is a difference in the matter of the shortening of the years between death [at the hands of the Heavens] and excision. For [in the case of] excision, the death of the person is before fifty years, whereas [in the case of] death, [it] is before sixty - like those that died in the desert. This is meaning to say, that one about whom it was decreed to live for seventy or eighty years, but became liable for death [at the hands of the Heavens], will die at less than sixty years. However there are righteous ones the [decree for which] is to have less days than sixty [years] - as they said (Taanit 5b), “One [who dies] at fifty-two years, this is the death of Samuel of Rama.” And it is stated (Numbers 4:18), “Do not let the group of Kohathite clans be cut off from the Levites.” And our Rabbis, may their memory be blessed, expounded (Yerushalmi Bikkurim 2:1) from this that excision is before fifty years. Its explanation is, let not an accident be how you cause the tribe of the Kohatites to be cut off from the service of the Levites. For if you do not guard them - that they should not come to see when the holy [vessels] are covered - they shall be excised from the service of the Levites, and will die before they are fifty years; as it is stated (Numbers 8:25), “But at the age of fifty they shall retire from the workforce, etc.”

Another statement we find in the Midrash there is: "If someone whispers to you; was not Gershon the firstborn? Why did the Torah accord superior rank to Kehat and his descendants and counted the Gersonites only afterwards? – Answer such inquiry by pointing out that the Kehatites carried the Holy Ark which is the Holy of Holies, and because Aaron who performed service in the Holy of Holies was a descendant of Kehat." Gershon, by comparison, was holy, but not "holy of holies." In order to demonstrate that Gershon had not forfeited his status as the firstborn amongst his brothers, the Torah writes: "נשא את ראש בני גרשון". Just as the Torah employs the expression ראש, when it instructs the Kehatites to be counted, so it employs the same expression when the count of the Gersonites is to be taken. (This term is missing when the Merarites are being counted in 4,29). Seeing that the Torah uses the expression גם הם, "also them," when it introduces the count of the Gersonites, one might have interpreted this as proof that they are considered inferior to the Kehatites.