Output Age from custom date value in WP Job Manager Field Editor (requires PHP 5.3+)

functions.php

<?php
// ^ the <?php above should only be in your functions.php file ONCE, at the top

// The filter is field_editor_output_as_value_METAKEY
// where you need to replace METAKEY with the actual meta key you want to filter the output for
//
// Below the example meta key used is "candidate_dob"
add_filter( 'field_editor_output_as_value_candidate_dob', 'smyles_output_age_from_dob' );

function smyles_output_age_from_dob( $value ){
  // No need to process if empty value (0, false, null, etc)
	if( empty( $value ) ) {
    	return $value;
  	}
  
  	// REQUIRES PHP 5.3+ (you should already be using this, if not, you have bigger problems to deal with)
  	// @see http://stackoverflow.com/questions/3776682/php-calculate-age
  	$age = date_diff( date_create( $value ), date_create('now') )->y;
  
	return $age;
}