From what I've read it would seem that young children have a far better ability to absorb, to learn, to aquire skills and knowledge than when we're older. Most language learning is done very early. By that premise alone it makes sense to school people when they are young.
Also they have a greater ability to do as they are told than later! Less disruptive, less likely to answer back, less likely to question whats on offer...all that makes them better students, in a more efficient classroom
i disagree with your last statement. Uni students tend to be as a group VERY well behaved (well except for arts students but who counts them) when compared to say grade threes (thats what my mum teaches). Further more QUESTIONING what your told is vital to eductation. If you arnt taught to question and resurch to double and tripple check then it really IS indoctrination rather than eductation