$("#age-form").submit(function(){
  var day = $("#day").val();
  var month = $("#month").val();
  var year = $("#year").val();
  var age = 18;
  var mydate = new Date();
  mydate.setFullYear(year, month-1, day);

  var currdate = new Date();
  currdate.setFullYear(currdate.getFullYear() - age);
  if ((currdate - mydate) < 0){
    alert("Sorry, only persons over the age of " + age + " may enter this site");
    return false;
  }
  return true;
});