How to add gender and age variables to my BMI calculation?

I'm trying to build a calculator that works out the users BMI.

I've managed to get this far at the moment, however I would like to add the users age and gender to the calculation.

<!DOCTYPE html>

<html>

<html>

<head>

<title>BMI Calculator</title>

<script type="text/javascript">

    function computeBMI()

    {

        //Obtain user inputs

        var height=Number(document.getElementById("height").value);

        var heightunits=document.getElementById("heightunits").value;

        var weight=Number(document.getElementById("weight").value);

        var weightunits=document.getElementById("weightunits").value;

        //Convert all units to metric

        if (heightunits=="inches") height/=39.3700787;

        if (weightunits=="lb") weight/=2.20462;

        if (heightunits=="cm") height/=100;

        //Perform calculation

        var BMI=weight/Math.pow(height,2);

        //Display result of calculation

        document.getElementById("output").innerText=Math.round(BMI*100)/100;

        var output =  Math.round(BMI*100)/100

        if (output<18.5)

        document.getElementById("comment").innerText = "Underweight";

      else   if (output>=18.5 && output<=25)

        document.getElementById("comment").innerText = "Normal";

     else   if (output>=25 && output<=30)

        document.getElementById("comment").innerText = "Obese";

     else   if (output>30)

        document.getElementById("comment").innerText = "Overweight";

       // document.getElementById("answer").value = output;

    }

</script>

</head>

<body>

<h1>Body Mass Index Calculator</h1>

<p>Enter your height: <input type="text" id="height"/>

    <select type="multiple" id="heightunits">

        <option value="metres" selected="selected">metres</option>

        <option value="inches">inches</option>

        <option value="cm">cm</option>

    </select>

     </p>

<p>Enter your weight: <input type="text" id="weight"/>

    <select type="multiple" id="weightunits">

        <option value="kg" selected="selected">kilograms</option>

        <option value="lb">pounds</option>

    </select>

</p>

<input type="submit" value="computeBMI" onclick="computeBMI();">

<h1>Your BMI is: <span id="output"></span></h1>

<h2>This means you are: <span id="comment"></span> </h2>

</body>