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();
}

You are right - you declared a new use defined type (Name_pairs) and you need variable of that type to use it.

The code should go like this:

Name_pairs np;
np.read_names()

just add static keyword at the starting of the function return type.. and then you can access the member function of the class without object:) for ex:

static void Name_pairs::read_names()
{
   cout << "Enter name: ";
   cin >> name;
   names.push_back(name);
   cout << endl;
}

If you want to call them like that, you should declare them static.