<form action="handle.php" method="post">
<input type="checkbox" name="age[]" value="18|19|20|">From 18 to 20<br>
<input type="checkbox" name="age[]" value="21|22|23|24|25|26|27|28|29|30|">From 21 to 30<br>
<input type="checkbox" name="age[]" value="31|32|33|34|35|36|37|38|39|40">From 31 to 40<br>
<input type="checkbox" name="age[]" value="41|42|43|44|45|46|47|48|49|50">From 41 to 50<br>
<input type="checkbox" name="age[]" value="51|52|53|54|55|56|57|58|59|60|61|62|63|64|65|66|67|68|69|70|">Above 50<br>
<input type="submit" />
</form>

<?php
$dbhost = 'localhost';
$dbuser = 'root';
$dbpass = 'root';
$dbname = 'test';
$con=mysqli_connect($dbhost,$dbuser,$dbpass,$dbname);
if ($con->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

$age = $_POST['age'];
$result = $con->query("SELECT name FROM users WHERE age REGEXP '$age[]' ");
$row_cnt = $result->num_rows;
echo "Your search returned" . $row_cnt . "results";
?>