Warning: Cannot modify header information - headers already sent by (output started at /home/w3org/public_html/age/index.php:79) in /home/w3org/public_html/age/index.php on line 133

The error message is a bit misleading, actually your output already begins in line 1, as your php script is a mix of HTML and php and any HTML parts of such mixed scripts are already output.

A header requires to be sent first, no output is allowed in advance.

What you could do is use output buffering, start your script with a php section even before the doctype:

<?php ob_start(); ?><!DOCTYPE...

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Then in case you decide you want to send a Location header instead of the page, you do

.... else{
      ob_end_clean(); // clear the output buffer and don't output it
	 header('Location: http://www.runningprofiles.com/error.php');
                                                exit();
			}

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And at the end of the script, for the normal case of no Location header output the buffer with

<?php ob_end_flush(); ?>

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Bye, Olaf.

As an alternative to doing ob_start() which is not really what you want to be doing here you should be doing all your routing logic at the top of your page. In other words

<?php
if(isset($_POST['submit'])) {
	$day=isset($_POST['day']) ? $_POST['day'] : false;
	$month=isset($_POST['month']) ? $_POST['month'] : false;
	$year=isset($_POST['year']) ? $_POST['year'] : false;

	$age = (date("md", date("u", mktime(0, 0, 0, $day, $month, $year))) > date("md") ? ((date("Y")-$year)-1):(date("Y")-$year));
	if($age > 21) {
		header('Location: http://www.runningprofiles.com/error.php');
	}
	// Now you can put the rest of your logic here.
?>

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What you have done is put your routing in the middle of your output which is not good practice. Routing happens before you start rendering the view.

Turn on output buffering