Levites Serving in the Temple

Where can I find Rabbis discussion on the age of training and service of the Priest. Especially this quote: “Rabbi Yosef B’chor Shor echoes sentiments expressed by Ramban when he notes that ’25 is when lighter work begins, including singing. At age 30, heavier carrying kicks in, as strength kicks in (based on Avot 5:21). The rabbis also taught that at 25 he begins his training, while at 30 he begins to serve.'”