Albert Barnes Commentary Luke 2:42

"And when he was twelve years old, they went up after the custom of the feast;" — Luke 2:42 (ASV)

Twelve years old. All males among the Hebrews were required to appear three times a year before God, to attend the ordinances of religion in the temple, and it is probable that this was the age at which they first went up to Jerusalem (Exodus 23:14–17; Deuteronomy 16:16).

To Jerusalem. All the feasts of the Jews were held there. This was a journey from Nazareth of about 70 miles.

After the custom of the feast. This was according to the usual manner of the feast.