It depends on your family history and weight/size. For example if your on the skinnier size you probably won't be very developed. But that is completely okay! If your on the chubbier side you could be a larger cup size. If I were to pick an average cup size it would probably be 30A-38A for an 11 year old girl.
32A or 34A would be average bra size for a 12 year old girl.
34B
There is no average since each girl/woman is different and breast size is based on DNA.
32A
32B
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.
D
34A
average would be 32B
30- 36
1AA
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