Chin Reduction. In general, it is best to wait, when possible, for the body (and face) to finish growing before considering any cosmetic procedures. At 16, your face will likely continue to grow and this will affect your overall facial proportions. If chin reduction is done prematurely, then as the face continues to change, what may have been a good result intially, no longer looks good.
Chin. It depends. First you have to make sure the problem is not the jaw that is forward. In that case chin surgery will be a patchy job. Otherwise, by taking lateral x-rays you could find out if the growing has stopped or not. Girls most likely have completed their growing by that age. (unless there is a condylar hyperplasia). So be careful!