פירוש על במדבר 4:2

נשא את ראש וגו׳ TAKE THE SUM OF [THE SONS OF KOHATH] — Count those that are fit among them for the service of carrying, and these are the men from thirty to fifty years, for a person less than thirty has not yet attained his full strength, — hence they (the Rabbis) said (Avot 5:21) “a man of thirty has reached the age when he enters into full strength”, — as for one who is older than fifty, his strength gradually diminishes from then.

נשא את ראש בני קהת, "take the sum of the members of the Kehatites, etc." We have already explained in Parshat Ki Tissa that the act of elevating the Israelites, i.e. numbering them (Exodus 30,12) was performed through their handing over the half shekel which formed their ransom money for the sin of the golden calf and that this is the reason the Torah used the term כי תשא את ראש when you "lift the head" when describing their being counted. This is in contrast with the members of the tribe of Levi who had no need to pay a ransom for their soul seeing they had not been guilty of that sin. This is why the Torah introduced the instruction to count the Levites (3,15) with the words: פקד את בני לוי, "count the members of the tribe of Levi." If all this is correct, why did the Torah change its wording when it came to counting the Kehatites and employ the same term נשא when instructing Moses and Aaron to count them?

For the service of carrying. Rashi intends to explain the reason for the count: All those enumerated are normally counted from one month, from twenty or from thirteen years of age, but this number is not similar to any other. Therefore he explains “count those among them suited…” Re’m.