Age prompt for website

Basically I want anyone who goes to my website confirm they're over 18 with a simple yes or no button.
How can I do this?

 

<script type="text/javascript">
var agev = document.cookie;
if(agev!='age=1'){
if(confirm("Are you above 18 years?")) setCo(); else window.location.href = "http://yoursite.com/notallowed.htm";
function setCo(){
document.cookie = 'age=1; path=/'
}
</script>


Try this, might contain syntax errors, did not test it. and change yoursite.com to ur domain