Hi Ieme,
As long as there is no medical problems, nothing is wrong with your body. Said that, I completely understand how you feel (I wouldn't be on this site if I was all happy with my boobs, would I?). There is so much pressure for us girls to achieve a specific image of what is beautiful, and we feel so ashamed and insecure if we deviate too much from that ideal.
I was very flat at your age, and it did not improved much as I aged (my mom and all my female relatives have bigger boobs than I do, go figure). The only time I had boobs was during pregnancy and breastfeeding. Even though I was so flat, I was able to breastfeed all my three children without problems. So technically, for their natural function, nothing is wrong with my boobs either. But I always felt (still feel) like nature cheated on me, and was embarrassed to be seen naked and allowing anyone to play with my boobs, even after I married to a wonderful man who likes me how I am.
I wish I had found this site 12 year ago. It turns out that really are things that you can do to improve your boobs besides implants. It is not as dramatic or fast, but they do work.
I would recommend you start by taking a hormone test. For starters, maybe try an online one such as
http://www.johnleemd.com/store/resource_...etest.html
IF you can afford it, doing a hormone test with real dr would be more accurate.
Then start reading like crazy on this site, there is ton of info on the different things you can try to stimulate growth. Somethings work for some people and not for others. You'll have to find what works for you. I would advise you though to try first things like the noggleberry or brava, before messing up with you hormones. You are still pretty young and developing, I would hate if you would have problems latter. And who knows, maybe you will sprout D by 21 like Demon Lord Etna's sister!
Take care