Ajax in the User registration form with 'containers'

Hi

In the user registration form the default type is "container". However, according to the manual the "container" form elements cannot take "#ajax":

http://api.drupal.org/api/drupal/developer%21topics%21forms_api_referenc...

Could someone explain how I can make one element dependent on another without using states? For example something like this and email textfield is dependent on the age of the user:

function custom_form_alter(&$form, &$form_state, $form_id){
	if($form_id == 'user_register_form'){
		$form['field_birthdate']['und'][0]['#ajax'] = array(
			'callback' => 'custom_fecha',
			'wrapper' => 'fechax',
    		);	
		$form['field_parents_email']['#prefix'] = '<div id="fechax">';
		$form['field_parents_email']['#suffix'] = '</div>';				
	}
}


function custom_fecha($form, $form_state){
	$ageok = custom_agecheck($form_state['values']['field_birthdate']['und'][0]['value']);
	if($ageok){
		$form['field_parents_email']['und'][0]['value']['#type'] = "markup";
	}
	else{
		$form['field_parents_email']['und'][0]['value']['#type'] = "textfield";
	}
	$f['field_parents_email'] = $form['field_parents_email'];
	return $f;
}