well It depends when they start to devlop like i Started to devlop at 8 years old and my friend is just starting to wear a bra at 13 years old so it just depend when she starts!
I have probably about, hmm.... medium breasts for my age, I am 10 and just today I started to wear bras. I had to learn to wear them, because I am getting to tjhe stage where i can date, so I dont want anything to go wrong with my breasts on my first date. Hope I helped P-s- A girl can start to ear a bra at any age, as long as shess comfortable wearing one, if she is not comfortable, try her on a trining bra. Hang loose! Peace out!