Underdeveloped breasts at 17?

Okay, so it's not exactly a question about size because I'm a 32C-D and happy with it. I'm 17 years old and according to various diagrams, I am still in the breast development stage seen commonly in ages 14-15. I did get my period at 13, which I don't think is late, but my best friend and an ex-boyfriend have commented that my breasts looked pretty "young" (not in a good way!!) Can I expect them to become like adult breasts this year or something?