18.
The FDA and Italy have decided you should be 18 before doing cosmetic surgery to change your breast.
I am a girl who believes in girl power. I think an 18 year old girl in these informational times can be astute and well informed. BUT your teens is a time of immense change. Girls mature at different times. When did you get your period? Stop growing? Is your weight fluctating? (And if not yet, have you gone to college and gotten the freshman 15?)
Who you are and what you want can change dramatically.
18 is a sweet spot in that it tends to be between high school and college for many women. Even then, 18 year olds vary. Some are wise for their years; some not so much.
Clearly there are exceptions: Poland syndrome, asymmetry, etc. But I think girls need to grow into themselves first. The option to operate is always there in the future.
I like 18. (or older)
http://www.telegraph.co.uk/news/worldnews/europe/italy/6796292/No-more-breast-enlargement-for-girls-under-18-Italian-government-orders.html