1 Chronicles - Chapter 23

In Numbers 4 we see that Levites were 30 years old when they began their service:

"So Moses, Aaron, and the leaders of Israel listed all the Levites by their clans and families. All the men between thirty and fifty years of age who were eligible for service in the Tabernacle and for its transportation numbered 8,580."

In this chapter David, therefore, counts the Levites who are 30 years old or more:

"David summoned all the leaders of Israel, together with the priests and Levites. All the Levites who were thirty years old or older were counted, and the total came to 38,000."

Although Jesus was from the tribe of Judah, rather than Levi, he was a priest as well as a King and Luke says (in chapter 3) that he started his ministry at the age of thirty:

"Jesus was about thirty years old when he began his public ministry."