Quite often children were promised at a young age and married at a young age. Though you are right to point out puberty happens at an earlier age for us, it still doesn't change that the practice of marrying prepubescent girls occurred and was common in that area pre-Islam.
Also, although we are experiencing puberty at earlier ages thanks to being pumped full of sweet chemicals, I would like to point out that back then more was expected of children at earlier ages in most cultures.
I wouldn't say that, again you get back to the word "Bloom" being used, including with Mohammad's followers marrying young girls. A 6 year old surely hasn't experienced puberty, nor has a 9 year old girl even today having done so. Again there would be no reason for them to have sex with a girl that young as they would be unable to carry a offspring for them, that is unless they just wanted to have sex with a child. Not to mention the physical damage it would do to a young girl's body at that age.