大致思路就是，先判断年份是否相差18年，若大于18，则直接返回已满18岁。若等于18，则判断月份是否是同一个月份，依次判断是否为同一日。
直接贴代码：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<script>
window.onload = function(){
jsGetAge = function(){
var flag = false;
var IDBorn = brith.value;
var birthYear = IDBorn.substring(0,4);
var birthMonth = IDBorn.substring(4,6);
var birthDay = IDBorn.substring(6,8);
var now = new Date();
var nowYear = now.getFullYear();
var nowMonth = now.getMonth()+1;
var nowDay = now.getDate();
var yearDiff = nowYear - birthYear;
alert(yearDiff);
if(yearDiff < 18){
//失败提示，未满18岁 return false;
alert("未满18");
}else if(yearDiff==18){
if(nowMonth - birthMonth < 0){
//失败提示，未满18岁 return false;
alert("未满18 ");
}else if(nowMonth - birthMonth ==0){
if(nowDay - birthDay < 0){
//失败提示，未满18岁 return false;
alert("未满18 ");
}else if(nowDay - birthDay >= 0){
alert("已满18 ");
var flag = true;
}
}
}else if(yearDiff > 18){
//满18岁，flag = true;
alert("已满18");
var flag = true;
}
if(flag){
alert("已满十八岁，可以继续办理后续交易了");
}
}
}
</script>
<body>
<table cellpadding="10" cellspacing="1" width="70%" border="1" align="center"
<tr >
<td>
出生日期：<input type="text" name="brith" id="brith" align="center"/>
<button onclick="javascript:jsGetAge()"> 计算</button>
</td>
</tr>
</table>
</body>
</html>