ขอสอบถามเกี่ยวกับการเลือกวันเดือนปี แล้วขึ้นอายุอัตโนมัติคะ

<scritp>
 function getAge()
 {
	var day = document.frm.day;
	var month = document.frm.month;
	var year = document.frm.year;
	var d = "";
	var m = "";
	var y = "";
	var nowdt = new Date();
	var nd = parseInt(nowdt.getDate());
	var nm = parseInt(nowdt.getMonth());
	var ny = parseInt(nowdt.getFullYear());
	var age = document.frm.age;
	var ageYear = 0;
	var ageMonth = 0;
	
	for(i=0;i<day.options.length;i++)
	{
		if(day.options[i].selected){
			d = day.options[i].value;
		}	
	}
	
	for(i=0;i<month.options.length;i++)
	{
		if(month.options[i].selected){
			m = month.options[i].value;
		}	
	}
	
	for(i=0;i<year.options.length;i++)
	{
		if(year.options[i].selected){
			y = year.options[i].value;
		}	
	}
	
	if(d != "" && m != "" && y != "")
	{
		s = new Date(y, parseInt(m)-1, d);
		d = parseInt(s.getDate()); 
		m = parseInt(s.getMonth()); 
		y = parseInt(s.getFullYear());
		
		ageYear = ny - y; 
		if(nm > m)
		{
			ageMonth = nm - m;
		}else if(nm == m){
			if(nd >= d)
			{
				ageMonth = 0;	
			}else{
				ageMonth = 11;
				ageYear = ageYear - 1;
			}
		}else{
			ageMonth = m - nm;
			ageYear = ageYear - 1;
		}
		age.value = ageYear + "ปี " + ageMonth + "เดือน";	
	}else{
		age.value = "";
	}
	
 }
 </script> 
 <form action="#" method="post" name="frm">
 
 	วันที่
 	<select name="day" id="day" onchange="getAge()">
 		<option value="">-</option>
 	<?
 		for($i=1;$i<=31;$i++){
			echo "<option value=\"".$i."\">".$i."</option>";
		}
 	?>
 	</select>&nbsp;
 	เดือน
 	<select name="month" id="month" onchange="getAge()">
 		<option value="">-</option>
 	<?
 		for($m=1;$m<=12;$m++){
			echo "<option value=\"".$m."\">".$m."</option>";
		}
 	?>
 	</select>&nbsp;
 	ปี
 	<select name="year" id="year" onchange="getAge()">
 		<option value="">-</option>
 	<?
 		$thYear = date("Y") + 543;
 		for($Y=$thYear; $Y >= 2450; $Y--){
			echo "<option value=\"".($Y - 543)."\">".$Y."</option>";
		}
 	?>
 	</select><br/><br/>
	คุณอายุ:
 	<input type="text" name="age" readonly>
 	
 </form>