John Lightfoot on Luke 2:42

[And when he was twelve years old.] "Let a man deal gently with his son till he come to be twelve years old: but from that time, let him descend with him into his way of living": that is, let him diligently, and with severity (if need be), keep him close to that way, rule, or art, by which he may get his living.

At twelve years old, they were wont to inure children to fasting, from time to time, or from hour to hour; that they might be accustomed to it, and so be capable of fasting upon the day of atonement.

Christ being now twelve years old, applies himself to his proper work, to be about his Father's business.

Solomon, when 'twelve years old,'...judged between the two women.

"R. Chama saith, That Moses, when he was twelve years old, was taken from his father's house."