Prueba esto a ver si te sirve
<HTML>
<HEAD>
<META HTTP-EQUIV="Pragma" CONTENT="no-cache">
<META HTTP-EQUIV="expires" CONTENT="Wed, 26 Feb 1997 08:21:57 GMT">
<TITLE>Validar tu Edad</TITLE>
</HEAD>
<BODY onLoad="document.forms[0].day.focus();">
<div align="center">
<script language="javascript">
function EsAdulto(fecha_nacimiento)
{
if(!IsDate(fecha_nacimiento))
return false; var fecha=new Date(fecha_nacimiento);
var nyear=fecha.getFullYear();
var nmonth=fecha.getMonth();
var hyear=(new Date()).getFullYear();
var hmonth=(new Date()).getMonth();
var ndays=nyear*365 + (nmonth)*30;
var hdays=hyear*365 + (hmonth)*30;
return (hdays-ndays) > (18*365);
}
function IsDate(inString)
{
inString += " "; //add some padding so call is not made out of bounds
outString = "";
endString = "";
if (Trim(inString).length == 2)
return false;
if (inString.indexOf("/") == 0)
outString = "00/";
if (inString.indexOf("/") == 1)
outString = "0" + inString.charAt(0) + "/";
if (inString.indexOf("/") == 2)
outString = inString.substr(0,3);
endString = inString.substring(inString.indexOf("/") + 1,inString.length);
if (endString.indexOf("/") == 0)
outString += "00/";
if (endString.indexOf("/") == 1)
outString += "0" + endString.charAt(0) + "/";
if (endString.indexOf("/") == 2)
outString += endString.substr(0,3);
endString = endString.substring(endString.indexOf("/") + 1,endString.length);
endString = Trim(endString);
outString += endString;
if (isDate(outString.substr(3,2),outString.substr(0,2 ),outString.substr(6,4)))
return true;
return false;
}
function y2k(number) { return (number < 1000) ? number + 1900 : number; }
function isDate (month,day,year)
{
var today = new Date();
year = ((!year) ? y2k(today.getYear()):year);
month = ((!month) ? today.getMonth():month-1);
if (!day) return false
var test = new Date(year,month,day);
if ( (y2k(test.getYear()) == year) &&
(month == test.getMonth()) &&
(day == test.getDate()) )
return true;
else
return false
}
function Trim(inString)
{
while (inString.length > 0)
{
if (inString.substring(0, 1) != " ")
break;
inString = inString.substring(1, inString.length);
}
while (inString.length > 0)
{
if (inString.substring(inString.length - 1, inString.length) != " ")
break;
inString = inString.substring(0, inString.length - 1);
}
return inString;
}
function valida()
{ fecha = document.forms[0].day.value + "/" + document.forms[0].month.value+ "/" +document.forms[0].year.value;
if(!EsAdulto(fecha))
{
alert("Lo sentimos. Usted no cumple los requisitos");
document.forms[0].day.focus();
return false;
}
return true;
}
function Pass(I){
if (document.forms[0].day.value > 31) {
alert("Los días deben estar entre (1-31)");
document.forms[0].day.focus();
return false;
}
if (I == 2){
document.forms[0].month.focus();
document.forms[0].month.value='';
return true;
}
}
function Pass2(I){
if (document.forms[0].month.value > 12) {
alert("Los meses deben estar entre (1-12)");
document.forms[0].month.focus();
document.forms[0].month.value='';
return false;
}
if(I == 2) {
document.forms[0].year.focus();
document.forms[0].year.value='';
return true;
}
}
</script>
<p> </p>
<table width="500" height="380" border="0" cellpadding="0" cellspacing="0">
<tr>
<td><div align="center">
<table width="502" border="0" cellspacing="0" cellpadding="0">
<tr>
</tr>
</table>
<p><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><strong><br>
Para poder accesar a este sitio debe ser mayor de 18 años.<br>
Por favor ingrese su fecha de nacimiento. </strong></font> </p>
<p><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><strong>Día
/ Mes / Año</strong></font></p>
</div>
<form action="web_introduccion.html" onsubmit="return valida()">
<div align="center">
<p>
<input type="text" name="day" maxlength="2" value="" size="2" onKeyPress="Pass(this.value.length); if ((event.keyCode > 47) && (event.keyCode < 58)) event.returnValue = true; else return false;" onFocus="this.value=''">
<font size="2" face="Verdana, Arial, Helvetica, sans-serif"><strong>/</strong></font>
<input type="text" name="month" maxlength="2" value="" size="2" onKeyPress="Pass2(this.value.length); if ((event.keyCode > 47) && (event.keyCode < 58)) event.returnValue = true; else return false;">
<font size="2" face="Verdana, Arial, Helvetica, sans-serif"><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><strong> / </strong></font>
<input type="text" name="year" maxlength="4" value="" size="4" onKeyPress="if ((event.keyCode > 47) && (event.keyCode < 58)) event.returnValue = true; else return false;">
</p>
<p>
<input type="submit" value="Ingresar">
</p>
</div>
</form></td>
</tr>
</table>
</div>
</BODY>
</HTML>