13/14 year old.
I was 11
I was bought my first actual bra at age 11. The women in my family, on both sides, are bosomy, and I started developing early. By the time I was 13 I was in a 32D.
I got my breasts early. I wore a training bra and sports bra until I was in middle school as only changed to a real bra because they were getting bigger. So maybe around 12?
Ha! I was already a b cup by then. I was once ashamed of my breasts being big when I was young so I refused to wear anything but a sports bra.
I filled in a bit early. Probably around 9 or 10.