...You can essentially think of it as flipping a coin (50% of the time you get 18 and 50% of the time you get 19)...
 I am quite sure 18 and 19yo players are
 not evenly distributed in the draft. It is more like a
 2:1 ratio in favor of 19 year-olds. This is pretty easy to check actually.
17/48 18 year-old players seems normal.