Albert Barnes Commentary Luke 3:23

"And Jesus himself, when he began [to teach], was about thirty years of age, being the son (as was supposed) of Joseph, the [son] of Heli," — Luke 3:23 (ASV)

Jesus began to be, and so on. This was the age at which the priests entered on their office, Numbers 4:3, 47; but it is not evident that Jesus had any reference to that in delaying his work to his thirtieth year. He was not subjected to the Levitical law in regard to the priesthood, and it does not appear that prophets and teachers waited until that age to commence their work.

As was supposed. As was commonly thought, or perhaps being Levitically reckoned as his son.