What age for a training bra?

I am posting this because me and my fiance really disagree that the kids is mother of my step daughter has her in a training bra, she hasn't started growing anything yet (not even mosquito bumps) and she's only in 4th grade. I feel like that is way too young. Am I the only one!?!?!?!?!? And for those of you wondering no I'm not going to have her not wear it, even though I strongly disagree I don't want to make my stepdaughter feel horrible.