Adam Clarke on 1 Chronicles 23:3

Thirty years and upward - The enumeration of the Levites made in the desert, Num 4:3, was from thirty years upwards to fifty years. In this place, the latter limit is not mentioned, probably because the service was not so laborious now; for the ark being fixed they had no longer any heavy burdens to carry, and therefore even an old man might continue to serve the tabernacle. David made another ordinance afterwards; see on 1Chr 23:24-27 (note).