Làm cách nào để xác thực tuổi trong JavaScript?

Student Information Form

function valid[] 
{
var na = document.getElementById["nm"].value;
var ag = document.getElementById["age"].value;
var em = document.getElementById["eid"].value.indexOf["@"];
var mno = document.getElementById["pno"].value;
if[na==""||na==null]
{
alert["Enter The Name"];
return false;
}
else if[isNaN[ag]||ag100]
{
alert["The age must be a number between 1 and 100"];
return false;
}
else if[em==-1] 
{  
alert["E-mail ID is not valid"];
return false;
}
else if[mno==""||mno==null]
{
alert["Enter The Phone No"];
return false;
}
else if[isNaN[mno]||mno.length>10||mno.length