Some areas of the country do throw the weight in youth competitions. If you or one of your athletes is scheduled to throw in a youth meet, you will need to contact the meet director and check what size weight they will be using for the age group.
In the sense of performing as designed, most radar detectors do detect the presence of police radar in the immediate vicinity of a speeding vehicle. Whether or not that information will actually prevent a driver from receiving a speeding ticket remains to be seen, however.
They detect heat as it moves through their field of view. When the heat level changes in an area the detector assumes this is motion. If you put it behind glass, especially energy efficient glass, this will greatly reduce the amount of infrared that gets through to the sensor and in most cases will not trip the sensor.