I wear a 30AA and I am 11, so I guess that a 30A would be perfect on a 12 year old. Not that anything wouldn't be. It's all about your build. If you are tall and skinny, you will grow at a later rate than others because fat is sometimes mistaken and put into a bra.
If you are worried that you are too small, think of it this way: How stupid would you look with bigger boobs! They wouldn't suit you half as much as your natural ones do.
If you are worried that you are too small, think of it this way: How stupid would you look with bigger boobs! They wouldn't suit you half as much as your natural ones do.