Varies a lot
Almost all girls are "normal" sized (about 95% of healthy people are normal, by definition), but details are hard to come by.
For young teen
girls, bra sizes can range from about 30 to 36 with cup sizes in the a's and b. The "average" bra size would mean averaging all the bra and cup sizes for all the 14 year old girls in the United States . . . That answer is unknown.