cannot call member function without object

You need to instantiate an object in order to call its member functions. The member functions need an object to operate on; they can’t just be used on their own. The main() function could, for example, look like this:

int main()
{
   Name_pairs np;
   cout << "Enter names and ages. Use 0 to cancel.\n";
   while(np.test())
   {
      np.read_names();
      np.read_ages();
   }
   np.print();
   keep_window_open();
}