how old does a kids has to be for his will not go with the other parent?

Nevada has no "age of discretion" but a child's wishes may be taken into account if the child is determined to be of "suitable age and discretion."  In my experience, 7 is too young, really -- the dispute remains between the parents, and the question is WHY the child has the attitude he does -- which can cut either way.  For background, the relevant statutes, etc., see http://willicklawgroup.com/child-custody-and-visitation/.  For a more detailed review of your facts, circumstances, and options, schedule a consultation with a family law specialist.