I know there is the whole freedom of religion thing... but baptism has always struck me as a contract of sorts. "I promise to do this forever, and should I fail I will be cut off from everything I've ever known"
I don't think 12 year olds really know what they are committing to, or have the critical thinking skills to enter in to such an agreement. I don't think our laws should let them. of course I realize this is a religious arrangement that can't really be governed by the state.