You can implement [hook_element_info_alter()][1] to alter the definition of the form element given from [system_element_info()][2].

The default definition is the following one:

      $types['date'] = array(
        '#input' => TRUE, 
        '#element_validate' => array('date_validate'), 
        '#process' => array('form_process_date'), 
        '#theme' => 'date', 
        '#theme_wrappers' => array('form_element'),
      );

[form_process_date()][3] is the function that set the year sub-element. If you replace it with your own process function, or add it, you can alter that limit.  
The code used from the default preprocess function is the following one.

      // Output multi-selector for date.
      foreach ($order as $type) {
        switch ($type) {
          case 'day':
            $options = drupal_map_assoc(range(1, 31));
            $title = t('Day');
            break;
    
          case 'month':
            $options = drupal_map_assoc(range(1, 12), 'map_month');
            $title = t('Month');
            break;
    
          case 'year':
            $options = drupal_map_assoc(range(1900, 2050));
            $title = t('Year');
            break;
        }
    
        $element[$type] = array(
          '#type' => 'select', 
          '#title' => $title, 
          '#title_display' => 'invisible', 
          '#value' => $element['#value'][$type], 
          '#attributes' => $element['#attributes'], 
          '#options' => $options,
        );
      }

As alternative, you can implement [hook_preprocess_date()][4], which is executed when [theme_date()][5] is called. (In Drupal 7, every theme function executes preprocess hooks, not just a theme function using a template file.) Doing so, you can alter the form element the preprocess function gets in `$variables['element']`.  

I would rather use the latter method, even though both the methods would work; it is simpler, as it doesn't require two hooks.





  [1]: http://api.drupal.org/api/drupal/modules!system!system.api.php/function/hook_element_info_alter/7
  [2]: http://api.drupal.org/api/drupal/modules!system!system.module/function/system_element_info/7
  [3]: http://api.drupal.org/api/drupal/includes!form.inc/function/form_process_date/7
  [4]: http://api.drupal.org/api/drupal/modules!system!theme.api.php/function/hook_preprocess_HOOK/7
  [5]: http://api.drupal.org/api/drupal/includes!form.inc/function/theme_date/7