Primarily because you will not be fully physically mature at 14 years old. Also, if you are on the skinny side, you are likely to remain flat chested whilst you're skinny, since breast tissue is one of the first things to gain weight and one of the first things to lose weight if you diet.
I have assumed from the question that you're female. However if you're male, being flat chested at 14 is to be expected, to a certain extent...
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