“From thirty [years old and above]” (Numbers 4:35); from here they said: Thirty years old for strength (Mishna Avot 5:21). “Their counted, by their families, was two thousand seven hundred and fifty” (Numbers 4:36). “Their counted, by their families,” this is what they said, that the Ark would kill of the sons of Kehat, as when he counted them from one month old, they were eight thousand six hundred, but when he counted them from thirty years old, not even one third of them were found.
“These are the counted of the Kehatite families, all who served in the Tent of Meeting, whom Moses and Aaron counted according to the directive of the Lord at the hand of Moses” (Numbers 4:37). “These are the counted of the Kehatite families,” this is what they said, that the princes did not count with them at the directive of the divine speech, as the princes are not mentioned here. “According to the directive of the Lord at the hand of Moses,” they acted at the directive of the Lord, just as the Holy One blessed be He said to Moses that Aaron should count the sons of Kehat with him, as it is stated: “The Lord spoke to Moses and Aaron saying: Take a census of [the sons of Kehat]” (Numbers 4:1–2).
That is why it is stated: “At the hand of Moses,” as Moses said to Aaron, because Aaron was excluded from all the statements in the Torah that were stated to Moses.