the young men of the priesthood [pirḥei khehunna]? Is there a distinction between these different terms? The rabbis say: Yes, there is a distinction. There, with regard to the priests who keep watch in the Chamber of Avtinas and in the Chamber of the Spark, the Mishnah is referring to priests who have not reached the age at which they are eligible to perform the Temple service, and therefore the tanna calls them young priests. The older priests would not keep watch, as they preferred to perform the Temple service. Here, with regard to the priests who sleep in the Chamber of the Hearth in order to be ready to perform the morning rites, the Mishnah is referring to priests who have reached the age at which they are eligible to perform the Temple service, and consequently the tanna calls them the young men of the priesthood.
תנן התם: בשלשה מקומות הכהנים שומרים בבית המקדש: בבית אבטינס, ובבית הניצוץ, ובבית המוקד.
§ We learned in a Mishnah elsewhere (Middot 1:1): The priests would keep watch in 3 places in the Temple courtyard: In the Chamber of Avtinas, and in the Chamber of the Spark, and in the Chamber of the Hearth.
והלוים בעשרים ואחד מקומות: חמשה – על חמשה שערי הר הבית. ארבעה – על ארבע פנותיו מבפנים. חמשה – על חמשה שערי עזרה. וארבעה – על ארבע פנותיו מבחוץ. אחד – בלשכת הקרבן, ואחד – בלשכת הפרוכת, ואחד – אחורי בית הכפורת.
And the Levites would keep watch in 21 places, as follows: 5 upon the 5 gates of the Temple Mount; 4 upon the 4 corners of the Temple Mount, within the wall surrounding the Temple Mount; 5 upon the 5 gates of the Temple courtyard, and 4 upon the 4 corners of the Temple courtyard outside the courtyard wall, surrounding the Temple courtyard. One watch is observed in the Chamber of the Offering, where animals that had been checked for blemishes were held in readiness for sacrifice; one watch is kept in the Chamber of the Curtain, where the Curtain separating the Sanctuary and the Holy of Holies was woven; and finally, one watch is kept behind the Chamber of the Ark Cover, in the area between the Holy of Holies and the western wall of the Temple Mount.