Why is NaN not functioning for my while loop? [closed]

I'm not allowed to use functions for this exercice and I can't figure out why NaN is not working in the last while-loop. How should I use correctly? I know that I should use functions in order to solve this but I've been asked not to.

var answer = prompt("Welcome!\nDo you want to come on board?").toLowerCase();

while ((answer != "yes") && (answer != "no") && (answer != null)) 
{
    alert("Svara med antingen \"ja\" eller \"nej\"!");
    answer = prompt("Welcome!\nDo you want to come on board?").toLowerCase();
}

if (answer === "yes"){

        alert("Hi!");
        var answer2 = prompt("Have you ever sailed?").toLowerCase();

        while((answer2 != "yes") && (answer2 != "no") && (answer2 != null)) 
        {
            answer2 = prompt("Have you ever sailed?").toLowerCase();
        }
        if (answer2 === "yes"){

            alert("I have a last question for you.\nHow old are you?");

            var age = "";
            while((age == "") && (age != null) && || isNaN(age))
            {
                age = prompt("How old are you?");
            }
            if (age <= 14){
                alert("You're too!");
            }
            else if (age >= 15){
                alert("Welcome on board!");
            }


        }
        else if (answer2 === "no"){
            alert("I need experienced sailors!");
            }   
}

else if (answer === "no"){
        alert("Goodbye!");
    }