Christians assume that Jesus was about 33 years old when he died on Calvary. They make this assumption because Luke records Jesus was about 30 years old when he began his ministry and John records in his Gospel that Jesus attended three annual Passovers. However, John's gospel was not written to chronologize the historical events in the life of Jesus but to describe how Jesus was. In the Synoptic gospels Jesus clears the Temple near the end of his ministry. In the Gospel of John he clears the Temple near the beginning of his ministry. Therefore, we cannot calculate with certainty Jesus' age from the time Herod began building the Temple which was 19 B.C.
Irenaeus of Lyons was a highly esteemed Christian writer. He was very cautious about guarding the apostolic tradition. In his work, Against Heresies, Irenaeus wrote that he believed Jesus died when he was in his forties and his ministry had been around twenty years long. Irenaeus reasoned that the Jews would not have said to Jesus, "you are not yet fifty years old" if Jesus was only about thirty years old since one would then expect them to say something like, "you are not yet forty years old." There is another possible allusion in the gospel of John which Irenaeus does not mention.
The Jews then said, "It took forty-six years to build this temple, and will You raise it up in three days?" But He was speaking of the temple of His body. (John 2:20).
So the Jews said to Him, "You are not yet fifty years old, and have You seen Abraham? (John 8:58).
Another consideration is the words of Jesus in Capernaum when he said he came to proclaim, "the year of the Lord's favor." This suggests Jesus' ministry was only one year, or it is an allusion to a Jubilee Year. Luke also records that Jesus' ministry began in the fifteenth year of the reign of Tiberius. Tiberius' reign began in 14 A.D. so this would put the beginning of Jesus' ministry about 28 A.D. Luke records Jesus was about thirty years old when he began his ministry. In the Book of Numbers there is the suggestion that God required men to be thirty to fifty years old to do priestly service (Num 4:1-4). Joseph and David, types of Christ, were also thirty years old when they began their reign (Gen 41:46; 2 Sam 5:4). If we subtract 30 years from the fifteenth reign of Tiberius we arrive at about 4 B.C for Jesus' birth.
Another consideration for the age of Jesus is the 40 year - 40 week Biblical pattern. 40 weeks is a round number which is intended to signify a pregnancy period. Israel spent forty years from their deliverance of Egypt to crossing the Jordan. However, their wilderness wandering lasted 38 years. Let us also recall that Jesus was in sort of a spiritual wilderness between his resurrection and ascension, similar to his 40 days in the desert in his temptation. This number 40 reflects the time between conception and birth. When Jesus rose from the dead God said of him, "Today I have begotten you."
Herod began building the Temple around 19 B.C. If we add 46 years we arrive at 28 B.C. This coincides with John's account that Jesus cleared the Temple at the beginning of his ministry. However, this does not account for the length of time which may have passed between his baptism and the Temple clearing event, not does it account for the discrepancy with the Synoptic time of this event.
Both Matthew and Luke record that Jesus was born during the reign of Herod (Luke 1:5; Matthew 2). Roman history indicates Herod died in 4 B.C.
Luke also records that Jesus was born when Joseph and Mary were required to travel to Bethlehem for an empire wide census. This census was decreed by Augustus Caesar in 8 B.C. The problem here is that Luke records the census occurred when Kyrenius (Quirinius) was governor of Syria. However, according to Roman records this man was not governor of Syria until 6-7 A.D. Titus was governor from 9 to 8 B.C. and Saturninius from 8 B.C. to 6 B.C.
The Magi also came to see Jesus because they saw "his star" in the heavens. A triple conjunction of Jupiter and Saturn occurred in 7 B.C.