Insert between < head > and < / head >
Or Just in Site Page Top
Code
<?if($USER_ID$=0)?><script language="JavaScript">
{
year = prompt('How old are you?');
if (year >='18')
{ alert('Welcome to Site!!!') } else { alert('Sorry but you are not-18'), top.location.href="http://filmsbaze.ucoz.net/" } // by master
};
</script><?endif?>