Sean,
To be clear...
Are you really asking for a definitive answer as to why our 15 and older numbers are dropping? (albeit last years rose).
You are surprised that the SFGD doesn't know the answer? Does anyone know the real answer? There could be hundreds of reasons. Most of those would not include 6U wrestling. That study would involve asking 15-18 year olds if 6U was the reason that they quit. You have called that a "small data request".
Crazy...