See? I'm right.
Here is a guide for anonymous.
A cup = budding breasts. Not completely flat as is the common misconception; true DFC doesn't really exist after girls go through puberty. Two little hills, basically. No real definition.
B cup = average. Not flat, but not particularly large either- not enough to get the girl really noticed. Not much cleavage.
C cup = slightly above average. Usually one breast is a handful. She will get noticed for having larger breasts than usual. Her cleavage is usually noticeable.
D cup = Can be rather ridiculous, depending on the girl's constitution. Basically, you will know a d cup when you see it. Tama-nee related. Anyone bigger than that is heading into custom bra territory.
Age doesn't really have anything to do with anything; once they hit puberty, anything is possible. I have known 15 year olds with E cups, for example.
This has been your lesson in cup sizes with Anonymous. And remember, a true breast connoisseur loves and appreciates all breasts, and his favorite breasts are the breasts of the woman he loves.