It is usually unnecessary and even inadvisable to do this in QV. Selecting nothing is equivalent to 'overall' (with the possible exception of null values), and you can select all possible values easily by clicking on the list box and typing *.
The logic that you propose could potentially confuse the user - lets say they select overall, and then want to drill into 18-49. The selection will be ignored. In addition, you will need a triggers and macros to do this, adding to the complexity without adding to the value.
Logic will get you from a to b. Imagination will take you everywhere. - A Einstein