The implication of Numbers 4 is that priests began serving in the temple when they were thirty years old.
But what did they do before then?
Were they expected to train to be priests? If so, were they studying or performing minor roles or some form of apprenticeship?
Or were they expected to go into secular work to broaden their horizons before taking on a priestly role?
My reason for asking is based on Ezekiel, who was probably 25 (Ezekiel 1:1-2) when taken into exile in Babylon. Had he stayed in Jerusalem, he would have been a priest (Ezekiel 1:3) when he reached 30 (Ezekiel 1:1). However God called him to be a prophet (Ezekiel 2:3-5). Yet he was apparently able to support himself and his wife (Ezekiel 24:18) in Babylon. I'm wondering if he therefore learnt a secular occupation.
In addition, Nebuchadnezzar took the skilled people into exile (2 Kings 24:13-14) which further implies that Ezekiel would have had a skill useful in Babylon. Furthermore, others who were priests when the Babylonians returned were not so lucky (2 Kings 25:18-21), still further implying that simply being a priest was not a useful skill for the Babylonians.