It depends when they hit puberty. If it is really early many kind of big.
The average size for a 14 year old girl can vary greatly depending on individual factors like height and weight. It is best to measure for an accurate fit rather than relying on averages.
32A or 34A would be average bra size for a 12 year old girl.
34B
AA
There is no average since each girl/woman is different and breast size is based on DNA.
There really isn't an "average" size bra for a ceratin age group. Most girls, however, are about a A- to a B-Cup by the time they are 13 years old. Still, it is normal for a girl to be smaller or bigger than that.
32B
32A
D
average would be 32B
She could get a sports bra and they are comfortable that's what I had on I am a ten year old girl and have to where cup bras size triple A
34A
There is no average bra size for a 10-year-old girl because each girl develops differently; some slowly and some faster, some earlier and some later. Many girls will begin to go through puberty when they are 10, at which time they will notice changes in the nipple area of their breasts. That is a good time to start wearing a training bra. When your breasts have developed a form and grown larger, you will need a bra. Your mother or a trusted female adult can help you determine the right time to start wearing a bra, as well as the right size.