Jesus didn’t start his public ministry until he turned 30. Do you think this had anything to do with Numbers 4, where we read that a Levite needs to be 30 years old before he can serve as a priest? If this is not the case for Jesus, then maybe it is for John the Baptist, who would have been 6 months older than Jesus and was a Levite. (I’m assuming John wouldn’t have started preparing the way for the Lord until he was 30 and able to serve as a priest). Not that any of this makes a huge difference, but it is certainly interesting to understand more about these situations so that we can understand more about God.—G.H.
I have wondered that myself. But Luke tells us that Jesus was "about 30"—which almost certainly means he wasn't 30. Most NT scholars peg the age at which he started his public ministry at about 32. (Birth about 6 BC, death 35 years later, in 30 AD. A minority of scholars place his death in 33 AD.)
Junior priests I imagine needed years of training. That could explain the wait (Num 4:3, 23, 29, 35, 39, 43, 47; 1 Chron 23:3). I’m not sure this is the main reason they had to be 30 to serve officially. After all, they retired at age 50 (Num 4:23, 35, 39, 43, 47)—just when you’d think they were at the peak of their usefulness.
There is no indication John the Baptist served at the temple (as his father Zechariah did), but yes, if he did, he would have been at least 30 years old, and perhaps more respectable in the eyes of all the priests and Levites. In Jesus's case, we are not told specifically why he waited two decades after his appearance at age 12 (Luke 2). Further, despite his eternal priesthood (Heb 7:21, 24; Psalm 110:4), he wasn't from the tribe of Levi, so the O.T. regulation wouldn't have applied to him anyway.
My thoughts in the NT Character podcast on Jesus may be helpful for those interested in pursuing this and other issues.