well i can only give my anecdotal evidence .
i have three silvers , two of which have drop eye had them since 5 inch , they didn't get drop eye untill they were about 13 to 14 inch .
right around the time they started to get a little more strength and power to there swimming and jumping , compared to from when just babies .
one of which i seen go from no drop eye to instant drop eye after banging his head on lid . i also have noticed this fish and the other one with drop eye bang them selfs hard and the eye would drop more .
then after a little time it partially went back although not all the way back into socket .
the third one has always been far more layed back and calm , doesn't freak out or jump as much as the other two . it has no drop eye at all and is currently 16 to 17 inches long .
now just a couple weeks ago this fish ( with no drop eye ) did wack its self when i was doing some maintenance . i immediately looked and thought oh no ! come thunder take it easy i dont want you to get drop eye to lol.
interestingly she did have her one eye just ever so slightly drop , but it has gone back now and no sign of a drop this is why i think its caused by trauma .
not to say that they are not predisposed to drop eye .
im wondering if it may be more to do with the structure of the skull rather then , as others wonder from interbreeding or fatty diet .