What does the Torah mean when it writes "v'Lo Ya'avod Od" and continues "Vesheires es Echav ... "?
Rashi: It means that, at the age of 50, a Ben Levi retires from the Avodah of carrying, and returns to lock the gates, sing 1 and load the wagons.
Rashi (in Avos, 5:21): It means that, at the age of 50, the ben Levi returns to advise the (younger) Leviyim and to teach them how to carry out their duties. 2
See Mishnah there.
According to the Rashbam, who does not mention singing at all, the ben Levi only continues to guard the gates. Refer also to 8:24:2:3, where he applies the same distinction to the five years prior to the age of 30 as well.
Ramban: Which was the main Avodah, but which they synchronized with the Avodah of carrying (See Ramban DH 've'Od', who elaborates further with regard to the Avodas ha'Shir).
Rashi writes that after 50, he ceases carrying and returns to Shirah. The Sifri says only that he returns to guard and load (Ramban). In Chulin (24a), it says that only [in the Midbar] when they carried, they are disqualified from Shir, but not in Shilo! (Moshav Zekenim)
Rashi writes that after 50, he returns to Shirah. Above (4:47), he said that Avodas Avodah is Shir with cymbals, and the verse permits only until 50!
Refer to 4:47:151:1.
Rashi writes that after 50, he returns to load the wagons and sing. If so, Bnei Gershon and Bnei Merari are not disqualified at all through age!
Riva: Rashi means that Bnei Kehas return, i.e. cease carrying and begin loading. Bnei Gershon and Bnei Merari, from the beginning of their Avodah they load the wagons! When they pass 50, they cease loading, and sing and guard.
Rashi writes that after 50, he returns to locking the gates. There were no gates in the Mishkan!
Riva: Locking the gates applies to Shilo and the Beis ha'Mikdash. Loading the wagons applied in the Midbar.