Like others have said, there is no average. At 15, I had next to nothing. I was still wearing training bras I was so small, whereas my mother at this stage was roughly a C-D. My cousin (almost as of overnight) got breasts that were DD or larger. I have a friend who was an H cup by grade 7. It kinda depends on your genetics. Im currently 19, and on one side of my family are smaller more "average" sized breasts (roughly a B-C cup) whereas the other side of my family come from very large chested DD size or larger.
I too didn't vote, because there is no "average". You just are. For example, I've spent my life as an A-B, while my sister (at the same age) was a C-D. When my aunt was my age she had to have specially made bras because they were so big. Even to this day she has to have them specially made.
I didn't vote because there is no "average" size for girls. There is a huge variation that can happen for sizes ranging anywhere from 30A to over 40DDD, depending on the female's weight and some of her genetics. I was a around a 32A at age 15. You will typically have a very similar bust size to that of what your mom has, but your weight also plays a big factor as well. Breasts are basically fatty tissue and if you look at pro beach volleyball players, for example, they have very small breasts as their body fat levels are almost zero. If you are looking to decrease your bust size and are overweight, as you lose weight your breasts will decrease in size and that is typically one of the first places you do lose weight.