Here is my solution with switch case:

<?php
 
echo 'Please input the age: ' . PHP_EOL;
$age = intval(readline());
 
switch (true) {
    case $age < 2:
        echo 'baby';
        break;
    case $age >= 3 && $age <= 13:
        echo 'child';
        break;
    case $age >= 14 && $age <= 19:
        echo 'teenager';
        break;
    case $age >= 20 && $age <= 65:
        echo 'adult';
        break;
    case $age >= 66:
        echo 'elder';
        break;
    default:
        echo 'Not a human...';
}

Mine is with if + elseif + else:

<?php
 
/*
•   0-2 – baby;
•   3-13 – child;
•   14-19 – teenager;
•   20-65 – adult;
•   >=66 – elder;
*/
 
echo 'Input the age: ' . PHP_EOL;
$age = intval(readline());
 
if ($age <= 2) {
    echo 'baby';
} elseif ($age <= 13) {
    echo 'child';
} elseif ($age <= 19) {
    echo 'teenager';
} elseif ($age <= 65) {
    echo 'adult';
} else {
    echo 'elder';
}