There are two distint occasions where the age of Jesus was made in the bible and much deductions can be made from the historical recorded activities at that time;
1. When He was 12 years old – “And the child grew, and waxed strong in spirit, filled with wisdom: and the grace of God was upon him. Now his parents went to Jerusalem every year at the feast of the passover.
And when he was twelve years old, they went up to Jerusalem after the custom of the feast.
And when they had fulfilled the days, as they returned, the child Jesus tarried behind in Jerusalem; and Joseph and his mother knew not of it. ” Luk 2:41-43
2. When he was 30 years old – “And Jesus himself began to be about thirty years of age, being (as was supposed) the son of Joseph, which was the son of Heli,..” Luk 3:23
Two popular methods have been used to estimate the year of the birth of Jesus:
1. Based on the accounts of his birth in the gospels with reference to King Herod’s reign.
2. By working backwards from his stated age of “about 30 years” when he began preaching (Luke 3:23) in “the fifteenth year of the reign of Tiberius Caesar” (Luke 3:1-2)
The two methods indicate a date of birth before Herod’s death in 4 BC, and a date of birth around 2 BC, respectively. Thus, most scholars assume a date of birth between 6 BC and 4 BC.