A simple program to calculate the age of the person using JavaScript programming language.

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Program Listing

index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Simple Age Calculator in JavaScript</title>
    <style>
        *,html {
            box-sizing: border-box;
            margin: 0;
            padding: 0;
        }
        body {
            background: #fefefe;
            min-height: 100vh;
            display: flex;
            align-items: center;
            justify-content: center;
            flex-direction: column;
            font-family: sans-serif;
            font-size: 110%;
        }

        h1, h3 { text-align: center; }
        h3 { margin-bottom: 40px; }

        form {
            display: flex;
            flex-flow: wrap;
            justify-content: space-between;
            max-width: 500px;
            width: 100%;
        }

        .block {
            flex-grow: 1;
            width: 150px;
            text-align: center;
        }

        .block:last-of-type {
            margin: 0;
        }
        
        select, input, button, .output {
            display: block;
            width: 100%;
            border: none;
            outline: none;
            padding: 10px;
            background: #000;
            color: #fff;
            cursor: pointer;
            font-size: 15px;
            height: 40px;
            text-align: center;
        }

        button, .output {
            background: #673ab7;
            font-size: 110%;
            flex-grow: 3;
            width: 100%;
        }
        
        .output {
            background-color: aqua;
            color: #000;
            cursor: auto;
        }
    </style>
</head>
<body>
    <div class="text">
        <h1>Simple Age Calculator in JavaScript</h1>
        <h3>Jake R. Pomperada, MAED-IT, MIT</h2>
    </div>
    <form>
        <div class="block">
            <label for="month">Month</label>
            <select id="month" required>
                <option value="01">January</option>
                <option value="02">February</option>
                <option value="03">March</option>
                <option value="04">April</option>
                <option value="05">May</option>
                <option value="06">June</option>
                <option value="07">July</option>
                <option value="08">August</option>
                <option value="09">September</option>
                <option value="10">October</option>
                <option value="11">November</option>
                <option value="12">December</option>
            </select>
        </div>

        <div class="block">
            <label for="day">Day</label>
            <select id="day" required>
                <option value="01">1</option>
                <option value="02">2</option>
                <option value="03">3</option>
                <option value="04">4</option>
                <option value="05">5</option>
                <option value="06">6</option>
                <option value="07">7</option>
                <option value="08">8</option>
                <option value="09">9</option>
                <option value="10">10</option>
                <option value="11">11</option>
                <option value="12">12</option>
                <option value="13">13</option>
                <option value="14">14</option>
                <option value="15">15</option>
                <option value="16">16</option>
                <option value="17">17</option>
                <option value="18">18</option>
                <option value="19">19</option>
                <option value="20">20</option>
                <option value="21">21</option>
                <option value="22">22</option>
                <option value="23">23</option>
                <option value="24">24</option>
                <option value="25">25</option>
                <option value="26">26</option>
                <option value="27">27</option>
                <option value="28">28</option>
                <option value="29">29</option>
                <option value="30">30</option>
                <option value="31">31</option>
            </select>
        </div>

        <div class="block">
            <label for="year">Year</label>
            <input type="number" min="1000" max="2022" id="year" value="2022" required>
        </div>

        <button type="submit" id="btnCalculate">Calculate</button>
    </form>
    
    <script>
        document.querySelector('form').onsubmit  = (e) => {
            e.preventDefault()

            // Extract and collect data
            let month  = parseInt(document.querySelector("#month").value),
                day    = parseInt(document.querySelector("#day").value),
                year   = parseInt(document.querySelector("#year").value),
                button = document.querySelector("#btnCalculate"),
                output = document.querySelector(".output"),

                // Today's date
                today  = new Date(),

                // Birthdate
                bdate   = new Date(year, month - 1, day),
                
                // Calculate the difference of dates
                diff = today.valueOf() - bdate.valueOf(),

                // Get year/s
                ageYear = Math.floor(diff / 31536000000),

                // Get day/s then convert to milliseconds
                calcDayMs = Math.floor((diff % 31536000000) / 86400000),

                // Get month/s
                ageMonth = Math.floor(calcDayMs / 30)

                // Get day/s
                ageDay = calcDayMs % 30,
                bdayInfo = "Years:"+ageYear+" Months:"+ageMonth+" Days:"+ageDay

            const elem = document.createElement('div')
            elem.className = "output"

            if (bdate > today) {
                elem.innerText = "Invalid date selected - Please try again!"
            } else {
                elem.innerText = bdayInfo
            }

            // Remove if output element exists
            output ? output.remove() : null;

            // Post result below Calculate button
            button.after(elem)
        };
    </script>
</body>
</html>

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