William Burkitt on Luke 3:23

At thirty years of age, the priests under the law entered upon their public office; accordingly Christ stays the full time prescribed by the law, before he undertakes his public ministry, and he gives the reason for it. That he might fulfil all righteousness. Matt 3:15 That is, the righteousness of the ceremonial law, which required persons to be of that age, before they entered upon that office; and also enjoined them to be baptized or washed in water, when they undertook their office. See Exod 29:4

Learn hence, that whatever the law required in order to perfect righteousness, that Christ fulfilled in most absolute perfection, both in his own person, and also in the name of all believers.

Observe farther, the title given to Joseph here: he is called the supposed father of Christ. Joseph was not his natural father, though so supposed by the Jews; but he was his legal father, being married to the Virgin when our Saviour was born; and he was his nursing father, that took care of him, and provided for him, though Christ sometimes showed both his parents, that, if he pleased, he could live without any dependence upon their care. See Luke 2:49