验证是否为数字使用javascript的内置函数isNaN()。该函数接受一个字符串类型的参数，如果参数不是数字，isNaN()方法返回true，否则返回false
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<script language="javascript">
function check(){
var age = document.getElementById("age");
if(age.value==null||age.value==""){
alert("请输入年龄！");
age.focus();
return;
}
if(isNaN(age.value)){
alert("年龄必须为数字！");
age.focus();
return;
}
document.getElementById("myform").submit();
}
</script>
<body>
<form action="" id="myform">
<table align="center">
<tr>
<td>用户名：</td>
<td><input type="text" id="name"></td>
</tr>
<tr>
<td>密码：</td>
<td><input type="password" id="pwd"></td>
</tr>
<tr>
<td>确认密码：</td>
<td><input type="password" id="pwd1"></td>
</tr>
<tr>
<td>性别：</td>
<td><input type="radio" name="sex" id="man" value="m">男
<input type="radio" name="sex" id="woman" value="f">女</td>
</tr>
<tr>
<td>年龄：</td>
<td><input type="text" id="age"></td>
</tr>
<tr>
<td align="center" colspan="2">
<input type="button" value="提交" onclick="check()">
</td>
</tr>
</table>
</form>
</body>
</html>