Gravity Forms Pre-Submission age check and redirect

gistfile1.php

<?php 

// update form id # (2)
add_action( 'gform_validation_2', 'custom_validate_not_too_old_pre_save' );

function validate_old_enough_pre_save( $validation_result ) {
  $birth_date = $_POST["input_16"];
  $birth_date = strtotime( $birth_date[0] . '/' . $birth_date[1] . '/' . $birth_date[2] );
  
  // user is under 13
  if ( time() < strtotime( '+13 years', $birth_date ) ) {
    $validation_result["is_valid"] = false;
    echo '<script type="text/javascript">
      window.alert("You are too young to join this site.")
      window.location.href="http://www.pbskids.com";
	    </script>';
	}
	return $validation_result;
}