Javascript input - Extended with an email and phone number system

var firstName = document.getElementById("name1").value;
var age = document.getElementById("age").value;
var secondName = document.getElementById("name2").value;
var email = document.getElementById("email").value.indexOf("@");
var phone = document.getElementById ("phone").value.indexOf("+");
Submitted = "true";
if (firstName.length > 15) {
    alert("The name must have no more than 15 characters!")
    Submitted = "false";
}
if (isNaN(age) || age < 10 || age > 120) {
    alert("The age must be between numbers 10 and 120!")
    Submitted = "false";
}
if (secondName.length > 15) {
    alert("The name must have no more than 15 characters!")
    Submitted = "false";

}
if (email.length > 200) {
    alert("The email must have no more than 200 characters!")
    Submitted = "false";
}
if (phone.length > 20) {
    alert("The phone number must have no more than 20 numbers!")
    Submitted = "false";
}
if (Submitted == "false") {
    return false;
}