A selective service card does not in any way indicate the individual has been called on or voluntarily made the step into adulthood. Today's 18 year olds are predominately immature self involved individuals. Absent demonstrable evidence to the contrary, they are not prepared for the responsibility of adulthood.
A Selective Service card indicates the individual is recognized, by Congress, as a member of the US militia, being an able-bodied male citizen 17-45, registered as our Founding Fathers prototyped in the Militia Act of 1792. The individual may be called up at any time and pressed into front-line service post haste. As a free adult citizen so recognized, they are expected to act as such.