This is about as simple as it gets.
Put this at the top of the page where you want the warning to appear.
Code:
<script language="javascript">
var agePrompt=prompt("You must be over 18 years of age to enter this ADULTS ONLY website. Please type just the number of your age below...")
if (agePrompt>=18)
document.location="http://www.letswatchsomeporn.com/";
else {
alert('Sorry, too young, we can not let you in!')
document.location="http://www.google.com/";
}
</script>
The code for the age verification on the site you mentioned is more complex and would require you to make or have someone make the three required graphics since you can't just use the ones you see on their site.