Average bra size for 17 year olds Watch
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As far as your question goes, I have no idea B/C cup?
Google isn't very helpful, but 32B seems to be the most common answer, this could be completely wrong though
Each person is different. We all have a different body shape and size, so I don't see the point in worrying about the "average" for this.
Besides, since most people don't know how bra sizing works, it wouldn't be very accurate.
Ontopic: Well of course there is an average.. there is for everything I guess. But well they are all too different and since most people actually have the wrong size bra well we will never really know.
If you're a teenage boy, please don't worry about. Pretty sure size means sweet nothing to guys except the higher the letter the larger the boobs = good time
And for the slightly less likely, if you're transitioning from a male to a female, don't aim for an average, choose the size that fits your body the best. As Kabloomybuzz said, it's not likely that a survey has been taken so there is no real 'average' anyway, not really.
17 is pretty much an adult in terms of breast growth, so everyone has pretty much reached their final adult size. So everyone will be different. However from what I've seen the most common size does seem to be a B/C cup, with a couple exceptions.