דברי הימים א, פרק כ״ג, פסוק ל׳

A central part of the Levites' work in the Temple was providing musical accompaniment for the daily public schedule. A specific group of Levites was given the additional, dedicated task of standing in the Temple every morning and evening [ביאור שטיינזלץ]. Their purpose was to offer thanks and praise to God through vocal and instrumental music. This musical service was carefully timed to occur twice a day, taking place exactly when the regular morning and afternoon sacrifices were offered [מצודת דוד]. Because of the unique nature of this musical role, it was assigned exclusively to mature Levites who were at least thirty years of age [מלבי״ם].