A mother can best advice a child on when she needs to wear a bra based on what she looks like without one.
Children should not wear Bras until they need to.
I don't think there is a certain size or age at which a girl should wear a bra.
It depends on what she looks like without one.
There is not a certain size at which they should wear one. It depends on what she looks like, in other Words does she look as if she needs one.
It depends on her height and weight.
It depends on whether she looks as if she should be wearing one.
It depends on how well developed they are.
If she is desperate to begin wearing a bra, but does not really need to wear one, you could start her off with a training bra. They a made almost like a sports bra. Try Target or Walmart.