Where was Jesus?
Mary and Joseph accidentally left twelve-year old Jesus at the temple once. Jesus had stayed back to "be about his Father's business." It wasn't rebellion--Jesus was doing God's will at that moment.
Couldn't Jesus just have come to earth as a 30-year old, to skip the growing-up years?
Jesus came as a baby, and grew up like any other child, in order that he might be able to fully sympathize with us in all of our weaknesses. Also, he needed to live all those years to show that he was the perfectly righteous Son of God and therefore capable of being the perfect sacrifice for the sins of all mankind.