From The exact prescribed initiation procedure is followed, and the Levites are consecrated to G‑d — in stead of the firstborn who lost their hallowed status when they participated in the sin of the Golden Calf. Towards the end of this section we learn the Levite service age-requirements and retirement age.

 As a music lover, I have to point out the Passuk that explains that the Levi’im will do the work because they are “netunim netunim”, or “given given” to G-d as a replacement for the first-born. The Medrash explains why double language was used here, because the Levites were given to do the work, and given to sing. Apparently singing is so important that it had to be mentioned separately. It is known that music has a power to elevate souls higher than words, but the fact that G-d appreciates it as much is even more satisfying.