The box is of such power it knows internally by the man who built it and programed the out come and changeability of some mature personage; one who is legally able and can decide what is righteous for them.
Any changes that the box might agree to do to those under the legal age would be for them as a short moment of thrill, and revert back to their human self by the next rising of the sun.
Yet, to those of such an equated age, realizing they are grown and matured enough to make a life changing decision; these are fair game for the usage of the black box.
"Ask and it shall be granted, plead thereafter and you shall surely die!" A small caption printed around the red button, the on button, onward and to your predetermined fate.