怎么使用js来验证年龄是否大于18岁

怎样使用js来验证年龄是否大于18岁
<div class="renzhenglx_div">
  <div class="renzheng_left_div" style="line-height: 20px">
  生日：</div>
  <div class="renzheng_right_div">
  <select name="Date_Year" id="Date_Year" class="select_b_div" style="width: auto;">
  </select>&nbsp;年&nbsp;&nbsp;
  <select name="birthday_m" id="birthday_m" class="select_b_div" style="width: auto;">
  </select>&nbsp;月&nbsp;&nbsp;
  <select name="birthday_d"" id="birthday_d" class="select_b_div" style="width: auto;">
  </select>&nbsp;日
  </div>
  </div>

[解决办法]

HTML code

<div class="renzhenglx_div">  <div class="renzheng_left_div" style="line-height: 20px">  生日：</div>  <div class="renzheng_right_div">  <select name="Date_Year" id="Date_Year" class="select_b_div" style="width: auto;">  <script type="text/javascript">      var i;      for (var i = 1980; i <= 2000; i++) document.write('<option value="' + i + '">' + i + '</option>');  </script>  </select>&nbsp;年&nbsp;&nbsp;  <select name="birthday_m" id="birthday_m" class="select_b_div" style="width: auto;">  <script type="text/javascript">      for (var i = 1; i <= 12; i++) document.write('<option value="' + i + '">' + i + '</option>');  </script>  </select>&nbsp;月&nbsp;&nbsp;  <select name="birthday_d"" id="birthday_d" class="select_b_div" style="width: auto;">  <script type="text/javascript">      for (var i = 1; i <= 31; i++) document.write('<option value="' + i + '">' + i + '</option>');      function checkage() {          var y =parseInt( document.getElementById('Date_Year').value)          , m = parseInt( document.getElementById('birthday_m').value)-1          , d = parseInt(document.getElementById('birthday_d').value);          var nowday = new Date(), birthday = new Date(y, m, d)          , age = nowday.getFullYear() - birthday.getFullYear() - ((nowday.getMonth() < birthday.getMonth() || nowday.getMonth() == birthday.getMonth() && nowday.getDate() < birthday.getDate()) ? 1 : 0); ;          alert(age)      }  </script>  </select>&nbsp;日<input type="button" value="检查年龄" onclick="checkage()" />  </div>  </div>