This is what I believe, based on evidences,what the situation as far as child marriages go. See there was a time in many societies and among many religions, when it was perfectly acceptable for girls to be married at puberty. this was the norm many many years back. In fact my granny(who was an Indian) married around sixteen or maybe even younger. There were reasons for this as well. The low life span of men and women being one.( at least this is what the older people say)and the fact that back then kids were not considered kids after puberty and they were brought up accordingly.
However now times have changed. Children at even eleven and twelve, though they may reach puberty, lack the physical and mental maturity of the the young ones years back. so its not practical or advisable for such youngsters to be having sexual relationships which can in turn result in pregnancy and child birth. Allahu'alim.. but this is a fact today.