Paste the following code in your single.php file, within the loop. Edited the text on line 7 as desired.
<?
$ageunix = get_the_time('U');
$days_old_in_seconds = ((time() - $ageunix));
$days_old = (($days_old_in_seconds/86400));
if ($days_old > 365) {
echo '<div class="disclaimer">DISCLAIMER: this post is older than one year and may not be up to date with latest WordPress version.</div>';
}
?>
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