Personally, I think we are missing the years before Jesus' ministry in the Bible to keep us from using them to deny his divinity. I think, generally, the divorce of his mortality from his divinity has done more to attack how Jesus really was (divine AND mortal) than perhaps anything else, and since Christianity generally already has denied his mortality in all practical ways, a record of his early years would complete the process by being used to deny his divinity.
I believe if we saw his entire life, most people would see him as just another guy - exactly as the people did in his own country who saw him growing up and couldn't accept him as the Messiah. I believe the only "perfect" (whole, complete, fully developed) picture of him includes a God as actual man, but admitting the "man" would destroy the conception of "God" held throughout "mainstream" Christianity.
People nit-pick prophets. Can you imagine how we would nit-pick someone who claimed to be the Savior and Redeemer of mankind if we were able to see his childhood, adolescence and all other pre-ministry aspects of his life? I firmly believe most Christians would reject Jesus if he lived now the life I believe he lived then, since their conception of the Savior and Redeemer of the world doesn't match the man I believe he was - and, to be fair, I think that's true of more than a few LDS members, as well, in some ways.
1 hour ago