It depends on a few factors such as whether we’re talking natural, pre-surgery sizes only. It’s especially difficult to calculate the average bra size among teenagers, because puberty can hit at wildly different times.
At age 7, 15% of girls are already developing breast tissue. By age 8, that figure rises to 25%. And yet there are others who make it to 12 or 13 with no signs of breast growth at all. It’s perfectly possible for one teen to be wearing an ‘adult’ bra while many of her peers are still in a training bra, or not needing to wear a bra at all.
Well I think the average bra size for a 13 year old girl in the US is still not known and also varies over different types of bras, from the same brand. However, the average size of fully grown women is a 34–36 band, with a C-D cup, again, dependent on bras.