Albert Barnes on Luke 3:23

Jesus began to be, &c. This was the age at which the priests entered on their office, Nu 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 did not commence their work before that age.

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

(t) "son of Joseph" Mt 13:55, Jn 6:42