I'm trying to add an age verification for an alcohol company's website, and I'm code listed below. The problem is that if someone's age is under 21, it still lets them enter the website. What do I need to add so that it won't let them enter the site, and I could add a message that they are too young to view this website?

I have this code at the head of the page:

<script language="javascript">
function checkAge()
{
var min_age = 21;

var year = parseInt(document.forms["age_form"]["year"].value);
var month = parseInt(document.forms["age_form"]["month"].value) - 1;
var day = parseInt(document.forms["age_form"]["day"].value);

var theirDate = new Date((year + min_age), month, day);
var today = new Date;

if ( (today.getTime() - theirDate.getTime()) < 0) {
alert("You are too young to enter this site!");
return false;
}
else {
return true;
}
}
</script>

And here's the form:

<form action="http://www.mhdbud.com/index.html" method="post" name="age_form">
Day : <select name="day">
<option>1</option>
<option>2</option>
<option>3</option>
<option>4</option>
<option>5</option>
<option>6</option>
<option>7</option>
<option>8</option>
<option>9</option>
<option>10</option>
<option>11</option>
<option>12</option>
<option>13</option>
<option>14</option>
<option>15</option>
<option>16</option>
<option>17</option>
<option>18</option>
<option>19</option>
<option>20</option>
<option>21</option>
<option>22</option>
<option>23</option>
<option>24</option>
<option>25</option>
<option>26</option>
<option>27</option>
<option>28</option>
<option>29</option>
<option>30</option>
<option>31</option>
</select>

Month : <select name="month">
<option>1</option>
<option>2</option>
<option>3</option>
<option>4</option>
<option>5</option>
<option>6</option>
<option>7</option>
<option>8</option>
<option>9</option>
<option>10</option>
<option>11</option>
<option>12</option>
</select>

Year : <select name="year">
<option>2003</option>
<option>2002</option>
<option>2001</option>
<option>2000</option>
<option>1999</option>
<option>1998</option>
<option>1997</option>
<option>1996</option>
<option>1995</option>
<option>1994</option>
<option>1993</option>
<option>1992</option>
<option>1991</option>
<option>1990</option>
<option>1989</option>
<option>1988</option>
<option>1903</option>
<option>1905</option>
<option>1904</option>
<option>1903</option>
</select>

<input type="submit" name="senddate" value="Go" onClick="return checkAge()">