I got the solution from https://drupal.stackexchange.com/questions/50358/save-a-new-value-with-entity-metadata-wrapper-to-an-entity-field-which-is-an-arr.

I created two separate form fields.

    function createnode_form($form, &$form_state){ 
      $form['field_student_age'] = array(
        '#type' => 'textfield',
        '#title' => t('test'),
      );
    
      $form['field_student_age2'] = array(
        '#type' => 'textfield',
        '#title' => t('test2'),
      );

Then I stored them using the first method in the post quoted above.

    $ewrapper->field_student_age[] = $form_state['values']['field_student_age'];
    $ewrapper->field_student_age[] = $form_state['values']['field_student_age2'];

Rather than using `set` as I'd done with single values, I created a new entry for each value using `[] =`.