The Baptism of Jesus and Our Baptism  
Why Jesus was baptized when he became 30 old? And the Church asks us to be baptized when we are children.
 
Jesus did not need to be baptized, he is sinless. The reason for his baptism, we find it the what John the Baptist himself says: "The next day John saw Jesus coming toward him and said, "Behold, the Lamb of God, who takes away the sins of the world ...I did not know him, but the reason why I came baptizing with water was that he might be made known to Israel. John testified further, saying, "I saw the Spirit come down like a dove from the sky and remain upon him. I did not know him, but the one who sent me to baptize with water told me, "On whoever you see the Spirit come down and remain, he is the one who will baptize with the Holy Spirit. Now I have seen and testified that he is the Son of God" John 1: 29-24
 
The baptism of Jesus then was for the only above reason. He started then his public life in preaching the good news.
Jesus as all Jewish children enter the covenant of the God of the Old Testament with their circumcision. We enter the new convenient with our baptism.
In fact all Jewish children need to be circumcised by the order of God to Abraham.
Our baptism enters us into the new covenant of God.
In fact God said to Abraham "This is my covenant with you and your descendants after you that you must keep: every male among you shall be circumcised. Circumcise the flesh of your foreskin, and that shall be the mark of the covenant between you and me. Throughout the ages, every male among you, when he is eight days old, shall be circumcised ...If a male is uncircumcised ...such one shall be cut off from his people; he has broken my covenant" Genesis 17:9-14
How a child of eight days will broke the covenant, he does know nothing. It is said then to his parents. Jesus also was circumcised on the eight day and his circumcision not his baptism made from him part of the covenant with his people. Luke 2: 21
Our baptism then is the circumcision of the people for the new covenant as teaches us St. Paul in Col.2:11-13
 
In today feast we remember the day that we became part of God's people, the church. St. Louis of France (Louis IX 1226-1270) used to insist to celebrate his birth day on the day of his baptism, because in baptism we are born for eternal life, when with our human birth we are born to die.
 Renew the vows of your baptism during today mass not because the baptism of Jesus was a need for him, but because it reminds us of the dwelling of the Holy Spirit in us who have been baptized, in the name of the Father, of the Son and of the Holy Spirit.