Работа с классами, наследование и инкапсуляция

<?php

class Person {
    protected $name;
    protected $last_name;
    protected $age;

    public function __construct ( $name, $last_name, $age ) {
        $this->name      = $name;
        $this->last_name = $last_name;
        $this->age       = $age;
    }

    /**
     * @param mixed $age
     */
    public function setAge ( $age ) {
        $this->age = $age;
    }

    /**
     * @return mixed
     */
    public function getAge () {
        return $this->age;
    }

    public function greet () {
        return "Name: $this->name, Last Name: $this->last_name, Age: $this->age;";
    }
}

class Student extends Person {
    protected $course;

    /**
     * Student constructor.
     * @param $name
     * @param $last_name
     * @param $age
     * @param $course
     */
    public function __construct ( $name, $last_name, $age, $course ) {
        parent::__construct($name, $last_name, $age);
        $this->course = $course;
    }

    public function greet () {
        return parent::greet() . "\nCourse: $this->course;";
    }
}

class Teacher extends Person {
    protected $dep;

    public function __construct ( $name, $last_name, $age, $dep ) {
        parent::__construct($name, $last_name, $age);
        $this->dep;
    }

    public function greet () {
        return parent::greet() . "\nDepartment: $this->dep;";
    }
}

$person = new Person('Alex', 'Heathen', 15);
//$stud = new Student('Alex', 'Heathen', 15, 2);
//$teacher = new Teacher('Alex', 'Heathen', 15, 'Math');
$person->setAge(10);
echo $person->getAge()."\n";