I would like to sum the column age as shown below on my table but im not sure how to implement this.

total sum

I did the below:

if (isset($_POST['date1']) && isset($_POST['date2'])) {
    
    $sql = "SELECT * FROM user WHERE signup_date BETWEEN '".$_POST['date1']."' AND '".$_POST['date2']."'";
    $result = mysqli_query($conn, $sql);
    
    if (mysqli_num_rows($result) > 0) {
        
        $x = 0;
        while ($row = mysqli_fetch_assoc($result)) {

            $output = '<tr>
                            <td>'.++$x.'</td>
                            <td>'.$row["name"].'</td>
                            <td>'.$row["email"].'</td>
                            <td>'.$row["age"].'</td>
                            <td>'.$row["signup_date"].'</td>
                        </tr>';

            echo $output;
        }