OP, if you're a 17 year old girl, please don't worry about it! Size only matters for you to purchase the correct fitting bra. What is more important is your comfort and confidence with your body. It takes time, but please don't compare yourself to other girls, everyone is different for different reasons (eg, genes or diet).
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.