My teens were 6th grade before they needed one. My oldest is now 17 and could honestly do without one (much to her dismay!) My youngest is 5th grade, very thin, has absolutely no need for a bra and doesn't want one. I teach a 5th grade class at church and maybe ONE girl in there wears a bra (and could do without IMO, she just acts way older than her age!)
I am surprised to read that so many of your girls need them at 9 years of age. Wow!
I also agree that most women, teens and adults, have such big ones these days. I think it is a combination of people being heavier overall, padded and uplifting bras, and the prevalence of plastic surgery. Everyone has cleavage these days!!!!